Well, the performance you get from your RAM varies according to your Speed (mHz) and the Latency (CAS in case). To you know the performance you get, you need to divide the CAS Latency by the Memory Speed (mHz) and multiplicate for 2000 (There is an picture explaining it better). To make things easier, I've made this calculation of all the frequencies of memories through latency, for when you buy your memory, know exactly which brings you more performance. The results are given in nanoseconds, lower is better. Enjoy it.
DDR4 mHz by CAS Latency.xlsx