Every time I ask for a lead change my gelding rushes through them. What are some exercises I can do to help him stay at an even speed during the changes?

my mare does the same thing. sit up tall and sit deep and when you ask for the change, squeeze on the inside rein and pull up slightly like a little half halt. this will also help to make him pick himself up instead of dropping his shoulder.

Figure eights around barrels, make him take it slow. Joggle his mouth a bit to slow him down as release when he slows.

My gelding does that also, I have him go over a ground pole, go two strides, change, go another two strides to another ground pole.