[Bug]: Threading always seems slower than multiprocessing

[MilesCranmer]

For whatever reason, I am always able to obtain a higher CPU utilization from multiprocessing mode rather than multithreading mode. I'm not sure if this is a Julia issue or if SymbolicRegression.jl is spawning tasks incorrectly.

The difference can be quite significant; the default example maxes out at around 350% utilization on my 8-core M1, but with multiprocessing I can above 700%.