The laundry list of bad things that result from swings that are too long is endless, but the big one for the average golf nerd is loss of lag. A swing too long disconnects the arms and on top of that, the golfer can’t generate enough speed to deliver the club with…
“Forward shaft lean, resulting in compression of the ball.”
Boy have I learned to hate that lingo.
Many of the swings I see that are too long are a result of late wrist set because people are widening the arc, float loading or some other catch phrase that belongs in a spaghetti western, to achieve lag. The body senses this and extends the backswing to give more time for the wrists to set. Swing gets too long, upper body tilts toward target, then from lack of speed and steep angle of attack from tilting toward the target….early flip, no lag, no shaft lean, no compression.
The irony is thick enough to need a particle beam laser to cut.
To shorten the swing, you need to get the wrists to set a 90* angle between the left arm and shaft (right handed golfer) by the time the left arm gets parallel to the ground. This makes it easier to hit pitches and 1/2 or 3/4 wedges, as well.
Here is a drill that shows you how natural that 90* angle is for the body to produce.