Dennis, 120v motors running at 50v will only get 50/120 = 0.4 of the rated speed of these motors, so say yours were indeed rated at 2500rpm then because you are running 50v you could only expect to get 1000rpm and it may actually be less.
With your setup you would need 10 revs of the motor to move 1 inch so for 500IPM you would be 5000rpm which is impossible. It would seem to me that you do not have the steps per unit set correctly as 500IPM is not slow and you were saying it was.
As said however the max you can expect to get is 1000rpm of the motor which would equate to 100IPM, not fast but not deadly slow like you are describing either.
Do you know what count encoders you have?
Hood