I have done some experimenting.
When using OpenCL...didn't test without.
The best way to configure your servers is 1 fft-thread and total-cores-minus-1 server-threads.
The fft_threads function doesn't do anything above 1 when using opencl, or maybe the parameter isn't read at all.
But I did notice that the threads parameter under server section makes a real big difference and should be set at max-1 cores.
Do not set it at max, as then it could hinder other tasks and slowdown other sections of Linux.
Tested the entire day on my tiny-easy-to-overload J5040 and these settings are important.
Setting the threads to 1 instead of 3 in my case, makes the difference between 5MHz and 20MHz streaming with the RX888MK2!!!!
So if you have problems, make sure these are set properly... ;D