Most stepper motor drivers need tens or hundreds of microseconds (10's of thousands to hundreds of thousands of nanoseconds which is the unit used in the program) to respond to a change in a step or direction signal. For example, the cheap 6560 drivers need 90uS to step. The Linistepper needs a bit less. The Pololu drivers only need 1uS (!!) which is really unusual and which has kept people who use the Marlin or Sprinter firmware stuck on that driver since that firmware puts out a tiny narrow little pulse. Our THB6064AH based driver works with the 1uS step pulse even though the chip actually needs 2.3uS because we stretch the pulse a bit on the board.
RAMPS Merlin NEMA23 via THB6064AH 50volt 4amp stepper driver - YouTube
Anyway, the point is that different drivers need different step times and you need to find that in the drive documentation.
James hosts the single best wiki page about steppers for CNC hobbyists on the net:
http://www.piclist.com/techref/io/steppers.htm Disagree? Tell him what's missing! ,o)