What should we add?

Right now the default number of CPUs used with parallel_map is the number of physical CPUs available:

This was done at the time because benchmarking showed that there was minimal if any improvement from running parallel processes on logical cores (i.e. SMT or hyperthreading cores). However, this code was introduced ~5 years ago (see: #701), we should check that this assumption is still true on more modern hardware. We should check this on various CPU types and also different OSes (also remembering multiprocessing is disabled by default on macOS and windows, so it will need to be explicitly re-enabled to test this).