Does It Matter What Slot You Put Your Ram In

Choose your iMac model

If you're not sure which iMac you have, you can identify your iMac and then select it from the list below.

27-inch

How to check RAM slots available on Windows 10. To confirm the number of memory slots available on your computer or laptop, use these steps on Windows 10. Open Start.; Search for Task Manager. If you're a curious type, you can look at the memory settings, timings and voltages in the BIOS and see that they now match the labels on the side of the RAM sticks. You can also see these settings listed in the program called CPU-z under the Memory tab. Some people manually tinker with these settings to get even more speed appearing – a. Then it doesn't matter which RAM goes in which slot. If your computer can only take one gig of RAM, then you'll need to take the gig stick back and buy two 512s. Then, obviously, you'll have to remove the already-installed 256 stick, put a 512 in each slot, and then your computer will have a gig. Depends if you have to exceed the rated voltage on the lowest voltage specced ram. Verify the lower voltage ram can run at the higher voltage without damage and note if it gets hot. You may be able to mitigate higher heat with a fan on your memory, I use an 80mm silent fan on my memory as part of my build.

24-inch

21.5-inch

• iMac (Retina 4K, 21.5-inch, 2019)*
• iMac (Retina 4K, 21.5-inch, 2017)*
• iMac (21.5-inch, 2017)*
• iMac (21.5 inch, Mid 2014)*
• iMac (21.5-inch, Late 2013)*
• iMac (21.5-inch, Late 2012)*

20-inch

17-inch

* Memory is not removable by users on iMac (21.5-inch, Late 2012), iMac (21.5-inch, Late 2013), iMac (21.5-inch, Mid 2014), iMac (21.5-inch, 2017), iMac (Retina 4K, 21.5-inch, 2017) and iMac (Retina 4K, 21.5-inch, 2019). If the memory in one of these computers needs repair service, contact an Apple Retail Store or Apple Authorised Service Provider. If you'd like to upgrade the memory in one of these models, an Apple Authorised Service Provider can help. Before you schedule an appointment, confirm that the specific Apple Authorised Service Provider offers memory upgrade services.

Memory in iMac (21.5-inch, Late 2015) and iMac (Retina 4K, 21.5-inch, Late 2015) is not upgradeable.

iMac (Retina 5K, 27-inch, 2020)

Get memory specifications for iMac (Retina 5K, 27-inch, 2020), then learn how to install memory in this model.

Memory specifications

This iMac model features Synchronous Dynamic Random-Access Memory (SDRAM) slots on the rear of the computer near the vents with these memory specifications:

For optimal memory performance, DIMMs should be the same capacity, speed and supplier. Use Small Outline Dual Inline Memory Modules (SO-DIMM) that meet all of these criteria:

• PC4-21333
• Unbuffered
• Nonparity
• 260-pin
• 2666MHz DDR4 SDRAM

If you have mixed capacity DIMMs, see the install memory section for installation recommendations.

iMac (Retina 5K, 27-inch, 2019)

Get memory specifications for iMac (Retina 5K, 27-inch, 2019), then learn how to install memory in this model.

Memory specifications

This iMac model features Synchronous Dynamic Random-Access Memory (SDRAM) slots on the rear of the computer near the vents with these memory specifications:

Use Small Outline Dual Inline Memory Modules (SO-DIMM) that meet all of these criteria:

• PC4-21333
• Unbuffered
• Nonparity
• 260-pin
• 2666MHz DDR4 SDRAM

iMac (Retina 5K, 27-inch, 2017)

Get memory specifications for iMac (Retina 5K, 27-inch, 2017), then learn how to install memory in this model.

Memory specifications

This iMac model features Synchronous Dynamic Random-Access Memory (SDRAM) slots on the rear of the computer near the vents with these memory specifications:

Use Small Outline Dual Inline Memory Modules (SO-DIMM) that meet all of these criteria:

• PC4-2400 (19200)
• Unbuffered
• Nonparity
• 260-pin
• 2400MHz DDR4 SDRAM

iMac (Retina 5K, 27-inch, Late 2015)

Get memory specifications for iMac (Retina 5K, 27-inch, Late 2015), then learn how to install memory in this model.

Memory specifications

This iMac model features Synchronous Dynamic Random-Access Memory (SDRAM) slots on the rear of the computer near the vents with these memory specifications:

Use Small Outline Dual Inline Memory Modules (SO-DIMM) that meet all of these criteria:

• PC3-14900
• Unbuffered
• Nonparity
• 204-pin
• 1867MHz DDR3 SDRAM

For these 27-inch models

Get memory specifications for the following iMac models, then learn how to install memory in them:

• iMac (Retina 5K, 27-inch, Mid 2015)
• iMac (Retina 5K, 27-inch, Late 2014)
• iMac (27-inch, Late 2013)
• iMac (27-inch, Late 2012)

Memory specifications

These iMac models feature Synchronous Dynamic Random-Access Memory (SDRAM) slots on the rear of the computer near the vents with these memory specifications:

Use Small Outline Dual Inline Memory Modules (SO-DIMM) that meet all of these criteria:

• PC3-12800
• Unbuffered
• Nonparity
• 204-pin
• 1600MHz DDR3 SDRAM

Installing memory

The internal components of your iMac can be warm. If you've been using your iMac, wait ten minutes after shutting it down to let the internal components cool.

After you've shut down your iMac and given it some time to cool down, follow these steps:

1. Disconnect the power cord and all other cables from your computer.
2. Place a soft, clean towel or cloth on the desk or other flat surface to prevent scratching the display.
3. Hold the sides of the computer and slowly lay the computer face-down on the towel or cloth.
4. Open the memory compartment door by pressing the small grey button located just above the AC power port:
5. The memory compartment door will open as the button is pushed in. Remove the compartment door and set it aside:
6. A diagram on the underside of the compartment door shows the memory cage levers and the orientation of the DIMM. Locate the two levers on the right and left sides of the memory cage. Push the two levers outwards to release the memory cage:
7. After the memory cage has been released, pull the memory cage levers towards you, allowing access to each DIMM slot.
8. Remove a DIMM by pulling the module straight up and out. Note the location of the notch on the bottom of the DIMM. When reinstalling DIMMs, the notch must be orientated correctly or the DIMM won't fully insert:
9. Replace or install a DIMM by setting it down into the slot and pressing firmly until you feel the DIMM click into the slot. When you insert a DIMM, make sure you align the notch on the DIMM to the DIMM slot. Find your model below for specific installation instructions and notch locations:
   • iMac (Retina 5K, 27-inch, 2020) DIMMs have a notch on the bottom, slightly to the left of the centre. If your DIMMs are mixed in capacity, minimise the capacity difference between Channel A (slots 1 and 2) and Channel B (slots 3 and 4) when possible.
     
   • iMac (Retina 5K, 27-inch, 2019) DIMMs have a notch on the bottom, slightly to the left of the centre:
   • iMac (27-inch, Late 2012) and iMac (Retina 5K, 27-inch, 2017) DIMMs have a notch on the bottom left:
     
   • iMac (27-inch, Late 2013) and iMac (Retina 5K, 27-inch, Late 2014, Mid 2015, and Late 2015) DIMMs have a notch on the bottom right:
     
10. After you've installed all of your DIMMs, push both memory cage levers back into the housing until they've locked into place:
    
11. Replace the memory compartment door. You don't need to press the compartment door release button when replacing the compartment door.
12. Place the computer in its upright position. Reconnect the power cord and all other cables to the computer, then start up the computer.
    

Your iMac performs a memory initialisation procedure when you first turn it on after upgrading memory or rearranging DIMMs. This process can take 30 seconds or more, and the display of your iMac remains dark until it's finished. Make sure you let the memory initialisation complete.

For these 27-inch and 21.5-inch models

Get memory specifications for the following iMac models, then learn how to install memory in them:

• iMac (27-inch, Mid 2011)
• iMac (21.5-inch, Mid 2011)
• iMac (27-inch, Mid 2010)
• iMac (21.5-inch, Mid 2010)
• iMac (27-inch, Late 2009)
• iMac (21.5-inch, Late 2009)

Memory specifications

Use Small Outline Dual Inline Memory Modules (SO-DIMM) that meet all of these criteria:

i5 and i7 Quad Core iMac computers come with both top memory slots populated. These computers will not start up if only a single DIMM is installed in any bottom slot; these computers should operate normally with a single DIMM installed in any top slot.

Core Duo iMac computers should operate normally with a single DIMM installed in any slot, top or bottom. ('Top' and 'bottom' slots refer to the orientation of the slots in the pictures below. 'Top' refers to the slots closest to the display; 'bottom' refers to the slots closest to the stand.)

Installing memory

The internal components of your iMac can be warm. If you've been using your iMac, wait ten minutes after shutting it down to let the internal components cool.

After you've shut down your iMac and given it some time to cool down, follow these steps:

1. Disconnect the power cord and all other cables from your computer.
2. Place a soft, clean towel or cloth on the desk or other flat surface to prevent scratching the display.
3. Hold the sides of the computer and slowly lay the computer face-down on the towel or cloth.
4. Using a Philips screwdriver, remove the RAM access door in the bottom of your computer:
   
5. Remove the access door and set it aside.
6. Untuck the tab in the memory compartment. If you're replacing a memory module, gently pull the tab to eject any installed memory module:
   
7. Insert your new or replacement SO-DIMM into the empty slot, noting the orientation of the keyway of the SO-DIMM as shown below.
8. After you've inserted it, press the DIMM up into the slot. There should be a slight click when you seat the memory correctly:
   
9. Tuck the tabs above the memory DIMMs, and reinstall the memory access door:
   
10. Place the computer in its upright position. Reconnect the power cord and all other cables to the computer, then start up the computer.

For these 24-inch and 20-inch models

Get memory specifications for the following iMac models, then learn how to install memory in them:

• iMac (24-inch, Early 2009)
• iMac (20-inch, Early 2009)
• iMac (24-inch, Early 2008)
• iMac (20-inch, Early 2008)
• iMac (24-inch Mid 2007)
• iMac (20-inch, Mid 2007)

Memory specifications

These iMac computers have two side-by-side Synchronous Dynamic Random-Access Memory (SDRAM) slots in the bottom of the computer.

The maximum amount of random-access memory (RAM) you can install in each computer is:

You can use a 1 GB or 2 GB RAM module in each slot for iMac (Mid 2007) and iMac (Early 2008). Use 1 GB, 2 GB or 4 GB modules in each slot for the iMac (Early 2009).

Use Small Outline Dual Inline Memory Modules (SO-DIMM) that meet all of these criteria:

DIMMs with any of the following features aren't supported:

• Registers or buffers
• PLLs
• Error-correcting code (ECC)
• Parity
• Extended data out (EDO) RAM

Installing memory

The internal components of your iMac can be warm. If you've been using your iMac, wait ten minutes after shutting it down to let the internal components cool.

After your iMac has cooled down, follow these steps:

1. Disconnect the power cord and all other cables from your computer.
2. Place a soft, clean towel or cloth on the desk or other flat surface to prevent scratching the display.
3. Hold the sides of the computer and slowly lay the computer face-down on the towel or cloth.
4. Using a Philips screwdriver, remove the RAM access door in the bottom of the computer:
   
5. Remove the access door and set it aside.
6. Untuck the tab in the memory compartment. If you're replacing a memory module, untuck the tab and pull it to eject any installed memory module:
   
7. Insert your new or replacement RAM SO-DIMM into the empty slot, noting the orientation of the keyway of the SO-DIMM as shown above.
8. After you've inserted it, press the DIMM up into the slot. There should be a slight click when you seat the memory correctly.
9. Tuck the tabs above the memory DIMMs, and reinstall the memory access door:
   
10. Place the computer in its upright position. Reconnect the power cord and all other cables to the computer, then start up the computer.

For these 20-inch and 17-inch models

Get memory specifications for the following iMac models, then learn how to install memory in them:

• iMac (20-inch Late 2006)
• iMac (17-inch, Late 2006 CD)
• iMac (17-inch, Late 2006)
• iMac (17-inch, Mid 2006)
• iMac (20-inch, Early 2006)
• iMac (17-inch, Early 2006)

Memory specifications

For best performance, fill both memory slots, installing an equal memory module in each slot.
*iMac (Late 2006) uses a maximum of 3GB of RAM.

Installing memory in the bottom slot

The internal components of your iMac can be warm. If you've been using your iMac, wait ten minutes after shutting it down to let the internal components cool.

After you've shut down your iMac and given it some time to cool down, follow these steps:

Does It Matter What Slot You Put Your Ram Instead

1. Disconnect the power cord and all other cables from your computer.
2. Place a soft, clean towel or cloth on the desk or other flat surface to prevent scratching the display.
3. Hold the sides of the computer and slowly lay the computer face-down on the towel or cloth.
4. Using a Phillips screwdriver, remove the RAM access door on the bottom of the iMac and set it aside:
   
5. Move the DIMM ejector clips to their fully open position:
   
6. Insert your RAM SO-DIMM into the bottom slot, keeping in mind the orientation of the keyed SO-DIMM:
   
7. After you've inserted it, press the DIMM up into the slot with your thumbs. Don't use the DIMM ejector clips to push in the DIMM, as this may damage the SDRAM DIMM.There should be a slight click when you seat the memory fully.
8. Close the ejector clips:
   
9. Reinstall the memory access door:
   
10. Place the computer in its upright position. Reconnect the power cord and all other cables to the computer, then start up the computer.

Replacing memory in the top slot

After you've shut down your iMac and given it some time to cool down, follow these steps:

1. Disconnect the power cord and all other cables from your computer.
2. Place a soft, clean towel or cloth on the desk or other flat surface to prevent scratching the display.
3. Hold the sides of the computer and slowly lay the computer face-down on the towel or cloth.
4. Using a Phillips screwdriver, remove the RAM access door on the bottom of the iMac and set it aside:
   
5. Pull the two levers on each side of the memory compartment to eject the memory module that's already installed:
   
6. Remove the memory module from your iMac as shown below:
   
7. Insert your RAM SO-DIMM into the top slot, noting the orientation of the keyed SO-DIMM:
   
8. After you've inserted it, press the DIMM up into the slot with your thumbs. Don't use the DIMM ejector clips to push in the DIMM, as this may damage the SDRAM DIMM.There should be a slight click when you seat the memory fully.
9. Close the ejector clips:
   
10. Reinstall the memory access door:
    
11. Place the computer in its upright position. Reconnect the power cord and all other cables to the computer, then start up the computer.

If your iMac makes a tone after you install memory

iMac models introduced before 2017 might make a warning sound when you start up after installing or replacing memory:

• One tone, repeating every five seconds signals that no RAM is installed.
• Three successive tones, then a five-second pause (repeating) signals that RAM doesn't pass a data integrity check.

If you hear these tones, confirm that the memory you installed is compatible with your iMac and that it's installed correctly by reseating the memory. If your Mac continues to make the tone, contact Apple Support.

Confirm that your iMac recognises its new memory

Does It Matter What Slot You Put Your Ram Install

After you've installed memory, you should confirm that your iMac recognises the new RAM by choosing Apple () menu > About This Mac.

The window that appears lists the total memory, including the amount of memory that originally came with the computer plus the newly added memory. If all the memory in the iMac has been replaced, it lists the new total of all installed RAM. Floor plan.

For detailed information about the memory installed in your iMac, click System Report. Then choose Memory under the Hardware section in the left side of System Information.

If your computer doesn't recognise the memory or doesn't start up correctly, confirm that the RAM you installed is compatible with your iMac and that it's installed correctly by reseating the memory. If you continue to have issues with the memory in your iMac, contact Apple Support.

Choosing RAM can be daunting, because this decision may have a tangible impact on your user experience. How many browser tabs can you keep open, how many layers can you have open in Photoshop, and how fast will your game load? All of this is directly affected by your choice of RAM.

How does RAM work?

Let us use a busy highway as an analogy. The number of RAM sticks is kind of like the number of lanes on the road: the more sticks of RAM you have, the more lanes you have open, and the more cars the road can hold at a given time.

The size of your RAM, in this analogy, would be how many cars each lane can hold. And the speed of your RAM is like the speed limit sign posted on the highway.

Therefore, a stick of 8GB RAM with a clock speed of 2666mhz can process 8GB of data at a given time, and 2666mhz is the speed at which the information will be processed.

This is why adding more RAM will not necessarily make your computer faster: you're adding more lanes to the road and more car capacity, rather than increasing the speed limit. Adding more RAM may be useful for users in certain RAM-hungry situations (heavily tabbed Chrome users, 3D rendering/modeling program users, and recent console—e.g. PS3, Wii U—emulation users), but more RAM is almost never the answer as to why a computer is not running a game at your desired performance level.

How do you determine RAM latency?

This is where CAS comes in. To understand how fast your RAM actually is, you must look at its memory timing. It will be listed alongside the RAM with numbers presented in this format: 15-16-16-35.

The first column represents CAS latency, also known as 'Column Access Strobe.' This is the number of clock cycles that pass between when an instruction is given and when the information is made available. If you tell someone to 'duck!', CAS latency would be the delay between when you command them to duck and when the other person reacts.

But CAS does not exist in a vacuum, and must be put into a formula alongside other specifications in order to determine the true latency of your RAM.

To calculate the true latency of your RAM in nanoseconds, use the following formula:

(CAS latency/RAM clock speed) x 2000 = latency in nanoseconds

If you have a CAS of 15 and a clock speed of 2400mhz, this would be your true latency:

(15/2400) x 2000 = 12.5 nanoseconds

If you have a CAS of 17 and a clock speed of 2666mhz, this would be your true latency:

(17/2666) x 2000 = 12.75 nanoseconds

Thus, higher CAS timings can result in a higher latency even with higher clock speeds. However, that doesn't explain what happens in a situation where the true latency of two sticks of RAM are tied, despite them having different clock speeds (because the faster-clocked choice has a worse latency). In situations where that kind of tie occurs, the higher speed of RAM takes precedence over which has superior CAS latency. Thus, when comparing a stick of DDR4-3000 RAM with a CAS of 15 and a stick of DDR4-3600 RAM with a CAS of 18 (which would both have a true latency of 10 nanoseconds), the DDR4-3600 should be preferred.

Similarly, the comparison of CAS latency between RAM options of identical clock speeds is where CAS matters most. A stick of DDR4-3600 RAM with a CAS of 15 has a true latency of 8.33 nanoseconds, while a same-size/same-speed stick with a CAS of 19 has a latency of 10.56 nanoseconds. In this case, the DDR4-3600 RAM with a CAS of 15 is objectively better than the DDR4-3600 RAM with a CAS of 19.

How do you choose RAM?

Motherboards and CPUs are rated for the maximum amount and speed of RAM they can handle. If your motherboard and CPU is only rated up to 3600mhz, then you should only purchase or overclock RAM up to 3600mhz. However, a quick perusal will reveal that there's over a hundred variants of 3600mhz RAM available. If you narrow it down to the size of the RAM, perhaps to 16gb (2 x 8GB), you still have a few dozen options. This is where CAS (and cost) comes in.

For 16GB RAM at 3600mhz, you'll find options ranging from a CAS latency of 15 to 19. You'll also find the price has a range of nearly $100 USD difference, and that this difference closely correlates to CAS latency. For example, the G.Skill Ripjaws V DDR4-3600 with a CAS of 19 costs about $130 USD. By comparison, the G.Skill TridentZ DDR4-3600 with a CAS of 15 is roughly $230 USD. The lower the CAS, the faster the RAM—and consequently, the more expensive.

When deciding between RAM of different clock speeds, the RAM with the higher clock speed is superior; but when choosing between RAM of identical clock speeds, the RAM with lower CAS latency is faster. This is where the constraints of your budget must do battle with your desire for speed (as well as any aesthetic considerations such as RGB lighting).

Does It Matter What Slot You Put Your Ram Instrument

If you hear these tones, confirm that the memory you installed is compatible with your iMac and that it's installed correctly by reseating the memory. If your Mac continues to make the tone, contact Apple Support.

Confirm that your iMac recognises its new memory

Does It Matter What Slot You Put Your Ram Install

After you've installed memory, you should confirm that your iMac recognises the new RAM by choosing Apple () menu > About This Mac.

The window that appears lists the total memory, including the amount of memory that originally came with the computer plus the newly added memory. If all the memory in the iMac has been replaced, it lists the new total of all installed RAM. Floor plan.

For detailed information about the memory installed in your iMac, click System Report. Then choose Memory under the Hardware section in the left side of System Information.

If your computer doesn't recognise the memory or doesn't start up correctly, confirm that the RAM you installed is compatible with your iMac and that it's installed correctly by reseating the memory. If you continue to have issues with the memory in your iMac, contact Apple Support.

Choosing RAM can be daunting, because this decision may have a tangible impact on your user experience. How many browser tabs can you keep open, how many layers can you have open in Photoshop, and how fast will your game load? All of this is directly affected by your choice of RAM.

How does RAM work?

Let us use a busy highway as an analogy. The number of RAM sticks is kind of like the number of lanes on the road: the more sticks of RAM you have, the more lanes you have open, and the more cars the road can hold at a given time.

The size of your RAM, in this analogy, would be how many cars each lane can hold. And the speed of your RAM is like the speed limit sign posted on the highway.

Therefore, a stick of 8GB RAM with a clock speed of 2666mhz can process 8GB of data at a given time, and 2666mhz is the speed at which the information will be processed.

This is why adding more RAM will not necessarily make your computer faster: you're adding more lanes to the road and more car capacity, rather than increasing the speed limit. Adding more RAM may be useful for users in certain RAM-hungry situations (heavily tabbed Chrome users, 3D rendering/modeling program users, and recent console—e.g. PS3, Wii U—emulation users), but more RAM is almost never the answer as to why a computer is not running a game at your desired performance level.

How do you determine RAM latency?

This is where CAS comes in. To understand how fast your RAM actually is, you must look at its memory timing. It will be listed alongside the RAM with numbers presented in this format: 15-16-16-35.

The first column represents CAS latency, also known as 'Column Access Strobe.' This is the number of clock cycles that pass between when an instruction is given and when the information is made available. If you tell someone to 'duck!', CAS latency would be the delay between when you command them to duck and when the other person reacts.

But CAS does not exist in a vacuum, and must be put into a formula alongside other specifications in order to determine the true latency of your RAM.

To calculate the true latency of your RAM in nanoseconds, use the following formula:

(CAS latency/RAM clock speed) x 2000 = latency in nanoseconds

If you have a CAS of 15 and a clock speed of 2400mhz, this would be your true latency:

(15/2400) x 2000 = 12.5 nanoseconds

If you have a CAS of 17 and a clock speed of 2666mhz, this would be your true latency:

(17/2666) x 2000 = 12.75 nanoseconds

Thus, higher CAS timings can result in a higher latency even with higher clock speeds. However, that doesn't explain what happens in a situation where the true latency of two sticks of RAM are tied, despite them having different clock speeds (because the faster-clocked choice has a worse latency). In situations where that kind of tie occurs, the higher speed of RAM takes precedence over which has superior CAS latency. Thus, when comparing a stick of DDR4-3000 RAM with a CAS of 15 and a stick of DDR4-3600 RAM with a CAS of 18 (which would both have a true latency of 10 nanoseconds), the DDR4-3600 should be preferred.

Similarly, the comparison of CAS latency between RAM options of identical clock speeds is where CAS matters most. A stick of DDR4-3600 RAM with a CAS of 15 has a true latency of 8.33 nanoseconds, while a same-size/same-speed stick with a CAS of 19 has a latency of 10.56 nanoseconds. In this case, the DDR4-3600 RAM with a CAS of 15 is objectively better than the DDR4-3600 RAM with a CAS of 19.

How do you choose RAM?

Motherboards and CPUs are rated for the maximum amount and speed of RAM they can handle. If your motherboard and CPU is only rated up to 3600mhz, then you should only purchase or overclock RAM up to 3600mhz. However, a quick perusal will reveal that there's over a hundred variants of 3600mhz RAM available. If you narrow it down to the size of the RAM, perhaps to 16gb (2 x 8GB), you still have a few dozen options. This is where CAS (and cost) comes in.

For 16GB RAM at 3600mhz, you'll find options ranging from a CAS latency of 15 to 19. You'll also find the price has a range of nearly $100 USD difference, and that this difference closely correlates to CAS latency. For example, the G.Skill Ripjaws V DDR4-3600 with a CAS of 19 costs about $130 USD. By comparison, the G.Skill TridentZ DDR4-3600 with a CAS of 15 is roughly $230 USD. The lower the CAS, the faster the RAM—and consequently, the more expensive.

When deciding between RAM of different clock speeds, the RAM with the higher clock speed is superior; but when choosing between RAM of identical clock speeds, the RAM with lower CAS latency is faster. This is where the constraints of your budget must do battle with your desire for speed (as well as any aesthetic considerations such as RGB lighting).

Does It Matter What Slot You Put Your Ram Instrument

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When dealing in nanoseconds, the difference may not seem significant—and it can certainly be argued that, for the average user, the difference between a CAS of 15 and a CAS of 19 is not worth breaking the bank. But when choosing between RAM options within budget, you should choose the lowest CAS at a given speed for the best performance.