If your motors turn very slowly using your Mach3 USB board, then either the pulse from the board is being output at a slow rate or the motor drivers are set to a higher than expected microstepping:
The Mach3 USB board outputs the pulse rather than the computer, but Mach3 controls this pulse frequency within the motor tuning section. First, determine if the travel distance is correct with the computer and the physical travel of the machine.
If the travel is different, then your stepper motor driver's microstepping is incorrect, or the steps per inch/mm in the motor tuning within Mach3 is incorrect.
If the travel is correct, then simply change the velocity and acceleration parameters to your desired level. If Mach3 will not achieve the velocity you desire, then decrease your stepper motor driver's microstepping setting, readjust the step per inch/mm in mach3 and readjust the acceleration and velocity to your desired levels.
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With the Mach3 USB board, my stepper motors are moving or spinning slowly.