tl;dr: libc++ is just bad, libstdc++ and MSVC trade punches for first place, with the eyeball win going to the FSF.
Though really the performance gates on string-heavy code tend to be in the heap and not the string library itself.
Though really the performance gates on string-heavy code tend to be in the heap and not the string library itself.
Clang developers are not stupid, if they did that instead of copying what GCC or MSVC did (they both predate clang), there is a good reason, and my guess is that it is better aligned with how modern architectures and optimizers work. Anyways, that's a tradeoff, but on a modern high performance computer, intuitively, I would side with clang.
modern 64bit cpus has no less than 32bytes cacheline