In my experience, the best performance comes from setting the number of workers to the number of real CPUs that you have, which should be the default. The workers end up waiting on the compute resources and there's no appreciable performance improvement, just more processes taking up more system resources.
The other things to watch for are
- exceeding system RAM limits -- using virtual memory is many times slower than RAM, so you may go faster with fewer workers.
- Getting blocked on storage I/O -- CPUs sit idle waiting on files to be read or write, meaning that the problem is not CPU-bound.
To optimize this you need to watch the system as it's running (there are plenty of graphical monitoring tools built into a modern OS), or save perfomance logs and then analyze them (these utilities have existed for a while on all platforms)
Other optimizations exist from analyzing your code with profiling tools and looking for ways to make slow sections of code faster.
