XNView could also profit from it by putting in some multi-threading.
First place to add it might be thumbnail processing.
Even if it's just a single core system there might be performance gains as the OS will schedule-away the thread that is IOWait and continue with the other one(s) that got data (from disk) already.
Other places might be resizing/effects/filters.
PS: I know getting multi-threading right is damn hard

V 1.90 <x>