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Updated 27-March-2025 in About Memory

RAM (Random-access memory) is an essential component of your computer. It is the temporary work area for your applications and operating system when your computer is running, so the faster your RAM, the faster your overall system performance. 

Memory speed can be measured in several ways. One measure of speed is in Mhz, which is a measure of the clock speed, or frequency of the RAM. The higher the clock speed, the faster the RAM can operate.

Another measure is bandwidth, which measures how quickly the memory chips can move data.

This article will focus on memory latency and CAS speed and the relationship between them because this relationship is another way to measure system performance.  While the two are closely related, they're not connected in the way you might think. 

Here's how speed and latency are related at a technical level — and how you can use this information to optimize your memory's performance. 

Read our article on computer memory to find out how much memory your computer should have.

Latency is a measure of the time delay between when a command is entered and when the data is available.

What is CAS latency and the latency equation?

At a basic level, latency refers to the time delay between when a command is entered and when the data is available. Latency is the gap between these two events. When the memory controller tells the memory to access a particular location, the data must go through a number of clock cycles in the column address strobe (CAS) to get to its desired location and complete the command. Two main variables determine a module's latency:

  1. The total number of clock cycles the data must go through (measured in CAS latency, or CL, on data sheets)
  2. The duration of each clock cycle (measured in nanoseconds) 

Combining these two variables gives us the latency equation: 

latency (ns) = clock cycle time (ns) x number of clock cycles 

In the history of memory technology, as speeds have increased (which means clock cycle times have decreased), the CAS latency values have also increased.   However, because of the faster clock cycle, the true latency as measured in nanoseconds has remained roughly the same. By optimizing the balance between the maximum speed your processor is capable of and the lowest latency memory available within your budget, you can achieve a higher level of performance using newer, faster, and more efficient memory. 

How can I check the CAS latency (CL) timings of my RAM?

You can check the CAS latency (CL) timings of your RAM using Crucial tools. The Crucial System Scanner provides detailed information about your system's memory, including CAS latency. Here's how you can do it:

  1. Download the Crucial System Scanner: Visit the Crucial System Scanner page and click on "Start my free scan."
  2. Run the Scanner: Open the downloaded file (CrucialScan.exe) and run it. The scanner will analyze your system's memory and storage setup.
  3. View Results: After the scan is complete, you'll see a detailed report that includes your RAM's CAS latency, along with other useful information about your system's memory and storage.

The perception and the truth about latency

Perception

  • Many users believe CAS latency is an accurate indicator of real world latency performance
  • Many users also believe that because CAS latencies increase with speed gains, some of the speed gets nullified

Truth

  • Semiconductor engineers know that CAS latencies are an inaccurate indicator of performance
  • Latency is best measured in nanoseconds, which is a combination of speed and CAS latency
  • Both speed increases and latency decreases result in better system performance
     

Example: because the latency in nanoseconds for DDR4-2400 CL17 and DDR4-2666 CL19 is roughly the same, the higher speed DDR4-2666 RAM will provide better performance

Example: if the speed rating of a standard module and a gaming module is the same (i.e. DDR4-2666) but the CAS latencies are different (i.e. CL16 vs. CL19), then the lower CAS latency will provide better performance

The difference between the perception of latency and the truth of latency comes down to how latency is defined and measured.

The latency paradox

Latency is often misunderstood because of product flyers and spec comparisons. It's usually noted as CAS latency (CL), which is only half of the latency equation. Because CL ratings indicate only the total number of clock cycles, they don't have anything to do with the duration of each clock cycle, and thus, they shouldn't be extrapolated as the sole indicator of latency performance.

By looking at a module's latency in terms of nanoseconds, you can best judge if one module is, in fact, more responsive than another. To calculate a module's latency, multiply the clock cycle duration by the total number of clock cycles. These numbers will be noted in official engineering documentation on a module's datasheet.

What is RAM speed?

RAM speed is essentially how quickly your computer's memory can process data, measured in megahertz (MHz). Faster RAM speeds mean quicker data access, which can boost your computer's performance, especially for tasks that require a lot of memory like gaming or video editing. However, it's not just about speed; the size, type, and timings of the RAM also play important roles in overall performance. So, it's best to consider all these factors together to get the most out of your computer.

How to tell what your RAM speed is

You can also check your RAM speed on your computer by using the Crucial System Scanner tool. The Crucial System Scanner is a free tool that scans your system to provide detailed information about your memory and storage, including RAM speed. Here's how you can use it:

  1. Download the Crucial System Scanner: Visit the Crucial System Scanner page and click on "Start my free scan."
  2. Run the Scanner: Open the downloaded file (CrucialScan.exe) and run it. The scanner will analyze your system's memory and storage setup.
  3. View Results: After the scan is complete, you'll see a detailed report that includes your RAM speed, along with other useful information about your system's memory and storage.

This tool is safe to use and doesn't install anything on your computer. If you need further assistance finding CAS latency timings or RAM speed feel free to contact support.

 

Which is more important: RAM speed or CAS latency?

Based on in-depth engineering analysis and extensive testing in the Crucial performance lab, the answer to this classic question is BOTH! Speed and latency both play a critical role in system performance. So, when you’re looking to upgrade, we recommend:

  • Step 1: Identify the highest memory speed supported by both your processor and motherboard (including overclocking profiles)
  • Step 2: Select the lowest latency memory that fits within your budget at that speed, remembering that a superior (i.e. lower) latency means superior system performance

To find out more about how our products can help you with RAM speed and CAS latency, browse our Crucial memory products. Also, be sure to navigate to our Crucial Upgrade Selector or System Scanner to see what RAM is compatible with your computer.

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