Heh, this is fun. By slowing down utf-8 validation in rustc we're seeing up to 44% performance decrease in short programs. This isn't exact science, but if the algorithm were to be *improved* it could have positive improvements for shorter programs. https://perf.rust-lang.org/compare.html?start=ae66171b41b1d05128afd0d666475bc4ba617469&end=c93d8c28b4e86aee203bca6ba8f4d7ecb2045c0d …
This was an attempt at speeding things up gone wrong -- so the results ended up skewing the other way around. If I had to guess in *large* Rust programs a lot of time is spent borrow checking / trait resolving / MIR etc. More % time is spent validating input in smaller programs.