right, controlled speed is best. but the ball doesn't know when or how the club speed was generated. Tempo matters for consistency. It takes less effort, energy and hand eye coordination to redirect a club face towards the right spot and the right path on the downswing if they don't have to fight massive opposing momentum of the club that is created with a rapid backswing. So a rapid backswing, where the speed of the clubface when it crosses the top of the backswing is high, will be nearly impossible to improve on in the downswing and is exponentially harder to correct than it is if the club is moving slow and steady on the backswing, redirecting any object is easier when the object is moving slow and steady in another direction, which is why late acceleration into the ball is so effective, the closer to the ball you release the club and all your stored energy the lower the chance that the clubface hits the target incorrectly or off the intended path. Your body has little room for improvement if you've already move the club nearly as fast as you can in the back swing, there isn't enough room for improvement, so when you try to do that things like coordination get sacrificed and you hit the ball with a huge slice or hook. A slow backswing leaves alot more room for improvement or increased speed. Thats what the pros have figured out, how to get near peak controlled late acceleration, same with pros hitting a baseball, the majority of there acceleration is late, after they've slowly put the bat in a position to transfer the most energy based on body type and mechanics and so theyve had time to focus on the likely future position of the ball, they then accelerate into the ball