There are type systems which can track complexity (e.g. http://gallium.inria.fr/~fpottier/slides/fpottier-2011-01-jf...) but that's still different to performance; async/sync tells us things like "touches disk" or "touches network" but not "O(2^n) might take a really long time".

Alternately, an effect system can communicate "touches disk", can make functions which are polymorphic to effects (e.g. map touches the disk if the mapping function touches the disk), and can distinguish different effects, unlike async/sync being anything-or-nothing. IMO async is more of an implementation detail (as "may suspend/yield" is not a very precise or interesting effect), whereas effects would communicate the properties that ohgodplsno wants to know.