Thanks for the link, that's an interesting read. Actually the referenced PyMutex is a good old pthread_mutex_t, the same you'd use in C or C++. But I shouldn't have written so surely. Although uncontested locks are very fast, if the loop is tight enough, adding locks will be significant.

However, PEP 703 specifically points out that performance-critical container operations (__getitem__/iteration) avoid locking, so I'm still highly skeptical that those locks are the cause of the 30-50%.

https://peps.python.org/pep-0703/#optimistically-avoiding-lo...