“You know what the best thing about magic is?
Oh, I think I see the source of the confusion: We see the gun firing then change angles to see him being struck by it. You're assuming that this somehow literally indicates him not moving until after the sound effect, rather than merely being the film cutting from "Shot of action" to "Shot of reaction".
...It would honestly never occur to me to look at it that way.
A Flock of Sheep.
A Pack of Wolves.
An Inconvenience of Heroes.
But the movie even foreshadows that he is that fast so I do believe that is how they meant it to be taken.
They just probably did not want Matrix graphics in it. Kung Fu: The Legend Continues uses similar things to display speed.. Low budgets are normally the cause but I think the more "realistic" feel that The Watchmen was going for is the reason behind it.
The Avengers: It is less of a movie and more of a Religious Experience.
It will change the way you view... well, everything.