freelist feature causes slowdowns universally

Bug Description

I've been naively enabling freelist to enhance performance of my applications. Turns out there's not a single case it helped the performance. Turns out the change https://github.com/PyO3/pyo3/pull/4902 caused a bottleneck that completely waives any benefits of using freelist - it's better to remove the feature altogether.

I digged deeper.

Fable 5 conclusion was: Short answer: because since PyO3 0.23.5 (Feb 2025), every freelist push and pop takes a std::sync::Mutex — on all builds, including GIL builds — and the locking overhead now exceeds what the freelist ever saved. I reproduced your observation in the container with a controlled experiment; the sign genuinely flipped.

Which makes sense to me and I think this observation is good.

Steps to Reproduce

1. Use freelist
2. Measure

Backtrace

Your operating system and version

Linux 6.18

Your Python version (python --version)

Python 3.14

Your Rust version (rustc --version)

rustc 1.96.0 (ac68faa20 2026-05-25)

Your PyO3 version

0.28.3

How did you install python? Did you use a virtualenv?

uv

Additional Info

Before Fable was restricted, I managed to ideate some ideas on potential ways forward: https://claude.ai/share/c18f23ad-b07e-4ffe-a285-bd40e92b81dd Do whatever you want with that information.

Start by reproducing the slowdown with freelist enabled and measuring push and pop performance, then inspect the change in PyO3 pull request 4902. Compare the results across the reported PyO3, Python, and Rust versions; done requires an agreed change that restores a measurable benefit or removes the ineffective feature.