Hello,
The maximum velocity is limited to the maximum frequency that can be generated on the stepper output. In the case of the 7350 series boards, the hardware can generate a pulse up to 8 MHz (page A-3 of the "NI 7350 User Manual"). Thus, the max velocity we can achieve is 8,000,000 steps per second.
Regards,
Jesse D.
Applications Engineering
National Instruments