Computer Memory Speed and Latency

How your PC memory speed and latency affect performance

Woman Assembling Random Access Memory at Computer

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The speed of the memory will determine the rate at which the CPU can process data. The higher the clock rating on the memory, the faster the system is able to read and write information from the memory. All memory is rated at a specific clock rate in megahertz that matches the CPU's the memory interface speed. Newer memory-classifying methods now refer to them based on the theoretical data bandwidth that the memory supports.

Types of Memory Speeds

All the versions of DDR memory are referred to by the clock rating but, more frequently, memory manufacturers are starting to refer to the bandwidth of the memory. These memory types can be listed in two ways. The first method lists the memory by its overall clock speed and the version of DDR that is used. For example, you may see mention of 1600 MHz DDR3 or DDR3-1600 which essentially is just the type and the speed combined.

The other method of classifying the modules is by their bandwidth rating in megabytes per second. 1600 MHz memory runs at a theoretical speed of 12,800 megabytes per second. Thus DDR3-1600 memory is also referred to as PC3-12800 memory. Here is a short conversion of some of the standard DDR memory that can be found:

  • DDR3-1066 = PC3-8500
  • DDR3-1333 = PC3-10600
  • DDR3-1600 = PC3-12800
  • DDR4-2133 = PC4-17000
  • DDR4-2666 = PC4-21300
  • DDR4-3200 = PC4-25600

It's important to know the maximum speed of memory that your processor can support. For example, your processor may only support up to 2666MHz DDR4 memory. You can still use 3200MHz rated memory with the processor but the motherboard and CPU will adjust the speeds down to effectively run at 2666MHz. The result is the memory is run at less than its full potential bandwidth. As a result, you want to buy memory that best matches your computer's capabilities.

Latency

For memory, there is another factor that impacts the performance—latency. This value measures the amount of time (or clock cycles) it takes the memory to respond to a command request. Most computer BIOS and memory manufacturers list this as either the CAS or CL rating. With each generation of memory, the number of cycles for command processing increases. For instance, DDR3 generally runs between seven and 10 cycles. Newer DDR4 tends to run at nearly twice that with latency running between 12 and 18. Even though there is higher latency with the newer memory, other factors such as higher clock speeds and improved technologies generally do not make them slower.

The lower the latency, the faster the memory is to respond to commands. Thus, memory with a latency of 12 will be better than similar speed and generation memory with a latency of 15. The problem is that most consumers will not really notice any benefit from the lower latency. In fact, faster clock speed memory with slightly higher latency may be a bit slower to respond but offer a greater amount of memory bandwidth, which can offer better performance.