A stepper is pulsed to make it rotate and the pulsing rate is related to the Kernal Speed you are able to use with your computer. Mach shares time with Windows, thus,the rate that is acceptable varies with different computers. So when you go to tune the motors, the Max Velocity may be resticted by the acceptable Kernal Speed. To find what is acceptable... run the Driver Test. Additionaly some drives may limit the amount of pulsing and that in return can limit the
the motors rpm.
A stepper at high rpm has very little torque. For what ever you are trying to do, just make note that it is so much more important for the stepper to run reliably for the task than have some fast motor rpm.
FWIW,
RICH