I imagine in the old days the idea was along the lines of your thinking. But now, more accurately in my mind, its about what speed is the ball at when a batter is making a decision to swing or not.
In MLB the average pitcher is releasing the ball 54' or so from home plate. The best hitters need to start their swing when the ball is 30-25' from home. They say the first 7-10' after release is identifying the baseball exists and left the pitcher's hand. So the entire decision tree on pitch ID, projected location, and whether or not to swing happens between 45' and 30' feet in front of the plate for the best hitters in the world. Fascinating.
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If that's the case, I would guess what the ball is doing as it enters that zone is more important than after leaving the zone. Shortening the decision window would be the ultimate value of velocity. The ball doing something unexpected after the decision window probably has a lot more to do with spin rates, arm angles, grip, and other factors.
The cool stuff is the things pitchers do to shorten the decision window that don't involve velocity. Hiding the ball from the batter's eye for a longer period of time by arm action can have the same effect on the decision window as adding 4-5 MPH to the fastball. Really fun stuff.