If you compare Weeden and Keenum for example, from the time the ball is snapped (they both played basically out of the shotgun last year) to the time the ball leaves the hand of the QB on each different throw (whether it's a quick out, a slant, a swing pass, a deep out, a post, a corner route, whatever), Keenum got the ball out of there a whole lot sooner than Weeden - and he was still more accurate.
This is more about going through a progression & rhythm than release. When most people talk about release, the time starts from the motion to get the ball to the ready position to the time it is out of his hands.
A QB that holds the ball low... around chest level has to get the ball back behind his ear, then go through his forward motion. That takes time. A QB that holds the ball near his head should take less time.
Even if you have two QBs who hold the ball near the same position, one is naturally going to be faster than the other.
Then you have those guys with the big baseball wind-up...