In general, if a PC's motherboard is only specified for RAM up to a given core speed x, will that PC be faster with:

  • RAM of latency y capable of running at a maximum core speed >x or
  • RAM of latency <y capable of running at a maximum core speed of exactly x ?

I would have thought the latter, but Crucial's Memory Adviser tool advises the former. So, which of us is correct - me, or the machine?

(Here is a concrete example: I wish to upgrade a Toshiba Satellite Pro L300-155 laptop from its current 1GB RAM to 2GB Crucial RAM. The laptop's specifications are given here.

I see from those specifications that the laptop is designed for DDR2-667 Ram.

Crucial sells two compatible 2GB kits, priced exactly the same as each other:

It seems to me that of these two upgrade kits, the first kit would run slightly faster on the L300-155 than the second, because both will presumably be capped at DDR2-667 core speed (see laptop specs), but the second kit has more latency.

However, Crucial's Memory Advisor tool recommends the second kit.)

  • Generally speaking, the latency depends on the speed. At DDR2-667 speed, the latency can usually be set lower than it would be at DDR2-800. (I'm not sure if it's possible to set the memory timings directly on a laptop BIOS, though, and I'm not familiar with the process by which the hardware selects what timings to use.) – user55325 Jan 23 '13 at 3:54

CAS Latency (or CL or “access time”) that tells us how many clock cycles the memory module will delay in returning data requested by the CPU. A memory module with a CL 5 will delay five clock cycles to deliver a requested data, whereas a memory module with a CL 6 will delay six clock cycles to deliver it.

Memory module with CL=5 will be faster because of above mentioned reason.

If you are stuck only because you are confused between memory with CL 5 and 6, then probably you are thinking too much hard on this. The difference is only 1 clock cycle and you are not going to even notice it, as it is very very very short peroid of time.

Instead you can think of adding number of RAMs your motherboard's memory channel archicture supports. E.g. If your motherboard support dual channel architecture and you have only one RAM installed, you can add another one (with same clock speed, otherwise both rams will run with minimum clock speed available between them). In this case you will get performance from your RAM.

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