The delay between inputs and their having effect is currently too random to make for a playable game.
Did you try the desktop build? I checked the web build just now and the input lag was okay (still <60fps though), but I think I saw the issue you're talking about during one of my earlier tests. Will look into it.
Desktop has the same problem, medium and high jumps it does easily but 9 out of ten attempts at a short jump are interpreted as medium.