This. He times the punch perfectly and usually as a counter shot. It's not like he just runs in trying to left hook the **** out of everything. He knows just when to use it.
To be fair Khan was in the middle of throwing the worst, most amateurish, technically awful uppercut I've ever seen. Left himself completely open to that hook, or anything else Garcia wanted to throw.
his counter punch was meant to get you out of there. It doesn't matter on the speed if he timed it right. And in reality it wasn't that slow compared how much weight he loaded it up on that shot.