Ball Speed
How fast the ball leaves the clubface at impact - the truest predictor of distance, driven by clubhead speed and quality of strike.
Ball speed is how fast the golf ball is traveling immediately after impact, measured in miles per hour. It is the single best predictor of potential distance - better than clubhead speed alone - because it captures not just how fast you swung but how efficiently that speed transferred to the ball.
The link between clubhead speed and ball speed is called smash factor: ball speed divided by clubhead speed. A driver hit flush off the center of the face produces a smash factor near 1.5, meaning a 100 mph swing yields roughly 150 mph of ball speed. Miss the center and ball speed - and distance - drops even if your swing speed is unchanged.
This is why center contact matters so much. Two golfers with identical clubhead speed can have very different ball speeds and very different distances based purely on where they strike the face. It is also why forgiving clubs, which preserve ball speed on off-center hits, help most golfers more than faster swinging would.
In fitting, ball speed and the launch and spin that accompany it are the numbers that ultimately matter, because they describe what the ball actually does rather than what the club or body did.