That quote doesn't really address anything, however. It is the equivalent of saying 'if I had been a bit slower dodging to the side, I would have gotten hit.' This has nothing to do with how quickly a threat can be neutralized, because the comment is specific specifically to his situation and not representative of the overall channeling system. For example, is Rand saying that the arrow fired at him in one scene that he caught with Air or the speed of his fall from the sky was less than how quickly the knife was thrown at him? Probably not. He's just using a figure of speech because in that situation, he almost didn't catch the knife.