The basic explanation is the delay before it actually reads data from the memory (not exactly but close enough). True memory speed is a function of the frequency and the CAS Latency. I forget the exact equation for calculating it is, but practically speaking 3200MHz CL16 and 3600MHz CL18 are almost identical. For real world numbers, the difference in performance between 3600MHz CL16 and CL18 are about 5% depending on the workload. For an iGPU system, faster memory is more important, so 3600MHz CL16 is about as slow as I'd recommend.
That makes sense. So what would you do about the fans I have? Do I necessarily need the controller for the RGB to work?