If you would measure pure latency between a single request (HTTP, RPC, whatever), the latency difference between any async or non async implementation should be microseconds at most and never milliseconds. If its more, then something with the implementation is off. And as you mentioned threads might even be faster, because there is no need to switch between threads (like in a multithreaded async runtime) or are there needs for additional syscalls (just read, not epoll_wait plus read).

async runtimes can just perform better at scale or reduce the amount of resources at scale. Where "at scale" means a concurrency level of >= 10k in the last benchmarks I did on this.