As far as I know, mechanical keyboards only make a difference in the type of keys they use and aren't anywhere different in speed. Regular keyboards use rubber plungers that when pressed to a circuit on a piece of plastic, cause an interrupt in the signal. With mechanical keyboards, all keys are individually microswitched (usually silent Cherry switches) and wired to a board.

Basically, typing on a mechanical keyboard feels like silk due to all keys being spring-loaded. While a regular keyboard feels like pressing... rubber?

And control speed in online games shouldn't differ from single player. During online play, your game and your character are being played live as in SP, just sending all movement & more to the server through packets. The quality of other players movement visible to you depend on latency.