The difference between the perception of latency and true latency comes down to how latency is ultimately defined and measured.
Latency is often misunderstood because on product flyers and spec comparisons, it's noted in CL, which is only half of the latency equation. Since CL ratings only indicate 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 true latency, multiply clock cycle duration by the total number of clock cycles. These numbers will be noted in official engineering documentation on a module's data sheet. Here's what these calculations look like.
In the history of memory technology, as speeds have increased, clock cycle times have decreased, resulting in lower true latencies as technology has matured, even though there are more clock cycles to complete. What's more, since speeds are increasing and true latencies are remaining roughly the same, you're able to achieve a higher level of performance using newer, faster, and more energy efficient memory.
At this point in the discussion, we need to note that when we say “true latencies are remaining roughly the same,” we mean that from DDR3-1333 to DDR4-2666 (the span of modern memory), true latencies started at 13.5ns and returned to 13.5ns. While there are several instances in this range where true latencies increased, the gains have been by fractions of a nanosecond. In this same span, speeds have increased by over 1,300 MT/s, effectively offsetting any trace latency gains.
Based on in-depth engineering analysis and extensive testing in the Crucial Performance Lab, the answer to this classic question is speed. In general, as speeds have increased, true latencies have remained approximately the same, meaning faster speeds enable you to achieve a higher level of performance. True latencies haven't necessarily increased, just CAS latencies. And CL ratings are an inaccurate, and often misleading, indicator of true latency (and memory) performance.
Optimise your system by installing as much memory as possible, using the latest memory technology, and choosing modules with as much speed as is cost-effective and/or relevant for the applications you're using.