I'll help the best I can. I work with motors quite a
bit. So it's all based on the number of "poles" in the magnetic field that drive the motor. More number of poles faster motor turns. Now all that said, it can also depend on current, strength of the stator, rotor clearance etc.

I would check a couple
of things to make sure your contacts are good. First look at the large threaded copper stud where
the batter cable goes. Make sure it's not worn down
or corroded. Rebuild houses are great for reusing
suspect parts to save a buck.