Why is bench_mp_pool so much slower from CPython 3.12 onwards?

[Fidget-Spinner]

Probably needs investigation: why is https://github.com/python/pyperformance/blob/008b5e3c65889c6328dad0a15185d233512f727e/pyperformance/data-files/benchmarks/bm_concurrent_imap/run_benchmark.py#L13

so much slower in CPython 3.12 onwards?

[eendebakpt]

According to the speed.python.org timeline

it around or just before python/cpython@23e812b (that is a documentation only PR, so seems unrelated).

Just before there is #101556 Change the multiprocessing start method default to forkserver, which might be related. @gpshead