In any case, even if decoding takes 50% of the time we spend in Emacs
code, it is still not significant enough to justify the un-safety of
using a string that we didn't decode, because if that string ever
includes raw bytes, Emacs will surely crash.
IMO if validation of the data that comes from external libraries is taking a lot of time then it does not make sense to use external libraries at all when we are aiming for performance gains.