They used to be instantanous, but chess.com changed it. I can't remember the reasoning behind the change but I do remember disagreeing with it.
Instant Pre-Move
I believe they said that in real life you couldnt possibly move that fast, so it wasn't fair to make it instant, but didn't want to take half a second away from you

Well, it's not possible to make a move in less than 0.1 secs without premoving, so I think it's fair for a premove to take 0.1 secs.
I agree, online chess isn't meant to be 'real life' chess. I would never play a bullet match in real life because I'd probably end up knocking all the pieces over and breaking the clock.
Instant pre-moves please.

In real life you also can't really commit your move without knowing your opponent's. I think that in exchange for this heightened risk profile, allowing moves to be made with no time penalty makes sense.
Is there any reason pre-moves take ~0.2s instead of being instant?
I find this very annoying especially when it comes to the end of bullet chess games. I've played on other servers and their pre-moves do not take any time off the clock.
Rob