Currently local games can be slowed down (by pressing '-') or speeded up (by pressing '+'). It would be generally nice but also benefit testing if it was possible to speed the game up further.
So I suggest to add at least one more speed step, accelerating games even more than the '+' key currently does.
Control could remain on + / - keys by default, with additional presses increasing / decreasing speed further. On keypress, the newly set speed step (such as: ultra slow (-3), very slow (-2), slow (-1), normal (0), fast (+1), very fast (+2), ultra fast (+3)) should be indicated on screen (as it already is now).