It depends. In a general purpose OS without scheduler trickery one would need at least some backoff & yielding syscall. And it's hugely workload dependent. But

It depends. In a general purpose OS without scheduler trickery one would need at least some backoff & yielding syscall. And it's hugely workload dependent. But more importantly, even with kernel backed waiting (e.g. using futexes) some userspace spinning can be important for performance. And then pretty much all the concerns from the spinlock apply there as well.