As some other commenters have mentioned, the GIL removal branch also makes some unrelated optimizations and the performance improvement comes from those rather

As some other commenters have mentioned, the GIL removal branch also makes some unrelated optimizations and the performance improvement comes from those rather than from the GIL removal itself, as I understand it. From your link: "Stripping out some of the GIL-removal related changes would result in even faster performance on the single-threaded pyperformance benchmark suite. [...] The resulting interpreter is about 9% faster than the no-GIL proof-of-concept (or ~19% faster than CPython 3.9.0a3). That 9% difference between the “nogil” interpreter and the stripped-down “nogil” interpreter can be thought of as the “cost” of the major GIL-removal changes." So it seems removing the GIL has a negative impact on single-threaded code, with the version that has both the GIL and the unrelated optimizations being 9% faster.