Add that it speeds up matching by 10-20%.

Please don't. These claimed speedups are highly misleading.

The release notes for 3.9 and 3.10 have various claimed large speedups, yet 3.9 is no faster than 3.8 and 3.10 only a little bit faster.

The benchmark results are convincing to me.

Without this it is not clear why bother with making this change at all.

You'll only see a 10-20% speedup for these specific benchmarks, which are contrived.
Real programs, even those that spend a lot regexes will spend a much lower proportion of their time in the regex library.

Any number we give will be misleading.

This branch vs 3.10:

- regex_v8: 147 ms +- 12 ms -> 132 ms +- 11 ms: 1.11x faster
- regex_effbot: 19.1 ms +- 1.2 ms -> 17.5 ms +- 0.9 ms: 1.10x faster
- regex_dna: 1.30 sec +- 0.01 sec -> 1.24 sec +- 0.01 sec: 1.04x faster

(Those numbers are very close to what you get by combining the latest re perf regressions with the numbers from this PR.)

I'll just leave the NEWS entry as-is (very few people read it anyways). I don't have a strong opinion on what should go in What's New (but @pablogsal might).

Perhaps the wording "up to 10% faster on the pyperformance regular expression benchmarks" (or similar) might suffice.

I think this should indeed go in the What's new. Could you add a small sentence in the Performance section, maybe? Specifying that the improvement is in the re engine and that it only affects re operations? If we need to remove it later is easier than the risk of forgetting to add something.