> As for mutex overheads, yep, that's annoying, but really, how annoying ?

For this use-case, you might not notice. ISTR, when examing the pthreads source code for some platform, that mutexes only do a context switch as a fallback, if the lock cannot be acquired.

So, for most use-cases of this header, you should not see any performance impact. You'll see some bloat, to be sure.