So here's the thing about Ryzen CPUs:
Inside the CPU there are chiplets containing physical cores, those chiplets and cores are connected with each other by a system called Infinity Fabric. This Infinity Fabric has its own clock and generally you want it to go as fast as possible. The faster the clock the faster cores can exchange data with other components inside the CPU package.
Now the Infinity Fabric clock, called FCLK works best when in sync with the memory clock (which is exactly half of the Effective Memory Speed). Generally Zen2 Ryzen CPUs can do about 1800MHz FCLK, meaning you want the memory clock to also be 1800MHz, meaning you want your effective RAM clock to be at 3600MHz.
If you use a 3200MHz RAM the memory clock (also called MCLK) and FCLK has to be lowered to 1600MHz directly affecting the speed of data exchange inside the CPU.
You can technically run FCLK and effective memory clock out of sync (for example 1800MHz FCLK and 3200MHz RAM) but that would cause huge latency penalty between RAM and CPU.
If you consider this and the fact that the memory timings don't impact in game performance by much it's clear that you should prioritize faster memory clock over lower Cas Latency (CL). Meaning 3600mhz cl18 is better than 3200mhz cl16.
Also as a side note 3200mhz cl16 will have almost exactly the same latency as 3600mhz cl18 (you can calculate the difference between ram kits by dividing memory frequency by cas latency and comparing the result, 3600/18 is equal to 3200/16)
