A function being marked async tells you nothing in particular about the performance of that operation. It could be anything from seconds to milliseconds and routinely is e.g. elliptic curve cryptography operations that are plenty fast enough to execute on a UI thread animating at 60fps are nonetheless marked async on the web, whilst attaching a giant document fragment to the live DOM - which might trigger very intensive rerendering calculations - isn't.

Nothing stops you from starting more threads when you run low in a scenario when there are only threads also. That's how the JVM ForkJoinPool works. If your threads end up all blocked, more are started automatically.