> Eventually somebody managed to make the Erlang code faster than the baseline C, then someone else made the C version 8k % faster, which proves your point. However, how is that related to using sync/async vs message passing?

If you want to write high-performance code then you need to be able to write synchronous code and have control over what your yield points are. If you take the "all calls are potentially async, runtime does what it wants" approach (i.e. "no function colouring") then you just will not be able to do that.