[TylerE]

Generally speaking async code in a language that isn't ground up designed around that is far slower, because the language ends up using some relatively expensive mechanism like mutexs. In most languages making 90% of your code 90% slower but "everything async" isn't really a worthwhile general trade off.

Even is your language run time is fully async, it's really hard to do anything useful without touching C/C++/the kernel/system libraries, and as soon as you do that all those guarantees go out the window, because you cannot rewrite the world in $newlang.