Stepper motor voltage / resistance per phase
With high step count stepper motors the inductance of the coils plays a big part in limiting the speed you can drive it at a given terminal voltage.







simulate this circuit &ndash; Schematic created using CircuitLab


Notice with the circuit above that the coil current has dropped to under a third of the nominal holding current at 12V resulting in less torque. Further there is a significant phase shift in the current which, in a closed loop system, will result in a further difference in applied torque if you do not correct for it in the switching angle.

This reduction of torque translates into a lesser top speed for any given shaft load. 

If your stepper driver is current regulated, using a larger supply voltage will reduce this effect and allow the motor to run faster for a given load.

Your other alternative is to source a motor with significantly less inductance.