> Synchronous sequences are generally faster than asynchronous ones because yielding and getting scheduled back has a non-zero overhead
Indeed. I remember being shocked when I first read "Thousands of Threads and Blocking I/O" [0] by Paul Tyma.
After reading the C10k problem [1], I just believed that a big pile of threads would never work and never updated my beliefs. While such an approach is still quite memory limited compared to asynchronous approaches, it scales quite far and can have a very high work rate.