Have you tried overlapping the signal, ie half-stepping? I know using this technique increases the torque of the motor but it could also speed up the motor.
Also, why not create your own "delay" function? Make a function with a simple loop in it and call that in b/n steps. To change the timing of the delay, increase or decrease the number of loops in the function. You'll have to tune this by hand, or if you have a frequency counter or oscilloscope to verify the speed of the pulses.