@fanf `tolower()` is the kind of function which can benefit from being inlined so maybe should have few adjustments there.

Ran on a AMD Ryzen 7 3700X so no AVX-512-BW sadly, using linux 6.6.21 + musl 1.2.5, (copy)tolowerin being musl's straightforward isupper+tolower functions inlined rather than linked to.
All compiled with Clang 18 and `-02 -march=native -mtune=native` in the CFLAGS.
vectolower.png