It's not the whole point, there's lots of other (albeit smaller) gains to be had once you have a strong async apparatus.

The core of your async implementation doesn't have to care about I/O - as long as it has a way to block/schedule fibers, it's easy to implement io_uring/IOCP based I/O on top of that - it's a matter of sticking a single IO poll in your main loop, and when you get a result, schedule the fiber that's waiting for it.

Another thing you get almost for free is an accurate Sleep(0.3) - your Sleep pushes the current fiber in a global vector with the time to be resumed, and you loop over that vector in your main loop.

We're writing a game engine so WaitForNextFrame() is another useful one - the implementation is literally pushing the current fiber to a vector and resuming it the next tick.