It's not like most of the calls in threads are generally high frequency calls.

create and async have to spawn a new interpreter and clone god know's what.join has to wait for a thread to finish and an interpreter to be cleaned up. Not much else is performance criticla either.

Even in the 'spawn and discard' model of server application, there are at least two IO operations which would totally swamp any overload overhead.

If the overloading had a detrimental affect upon some of the calls in threads::shared, like lock or cond_signal, that might be noteworthy, but presumably that is not the case?