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Here is patch that speed up *_find_max_char stringlib functions.
Microbenchmarks:
./python -m timeit -s "s='A'*20+'\x80'" "s[:-1]"
./python -m timeit -s "s='A'*20+'\U00010000'" "s[:-1]"
Results on Intel Atom N570 @ 1.66GHz:
vanilla patched
 0.819 0.73
 0.788 0.755

> 0.819 0.73
> 0.788 0.755
Are there any circumstances where this produces a larger speedup?
I don't think a 10% improvement on a micro-benchmark is interesting at all (it's basically 0% on any real-world program).

> Are there any circumstances where this produces a larger speedup?
I don't know. *_find_max_char in any case only a part of expensive
functions. Even if it is optimized to zero time, this will most likely
not produce a larger speedup.
> I don't think a 10% improvement on a micro-benchmark is interesting at all (it's basically 0% on any real-world program).
I agree. It is rather a question of purity. In my opinion the function
has become easier (for developer and for compiler).

Patch updated to tip.
But because results are contradictory and often patched version even slower vanilla (and I don't understand why it happens), I close the issue. I think all changes are irrelevant and differences are only compiler optimization artifacts.

Thanks for the effort, Serhiy!