Used excel to calculate the quick and dirty ram performance based on speed/CAS (explained by anandtech here
(This is very rough approx, and assumes other variables same (error), only looks at CAS and speed on performance, and not taking into effect architectural variables (error)). But again only way to currently get rough estimate, and ddr4 archit, etc differences may make slighlty faster than calc, but should not to be too far out of the ballpark.
The lower the latency in nanoseconds = the better/faster the performance, comparing both nonsequential reads (1 word speed) and sequential reads (8 word speed), since speed/timings have varying effects depending on seq vs nonseq.
A lot of common good XMP DDR3 is 2400 CAS 10 or CAS 11. You can see if you can get ddr4 3000+ at CAS 12 that will exceed common ddr3 performance of 2400 CAS 10, plus add small effects of quad channel, etc.