Your machine might not be rigid enough to move at those speeds . You can slow down your spindle to match your cut speed to get the exact same quality as you get at 100ipm. You might also have a setting for overshooting/undershooting. How square are your squares? To get the desired cut quality at the slower speeds use this formula. 100 ipm / your spindle speed. And then do 30 ipm/your spindle speed. Lower the spindle until you get a number that's close enough to the 100 ipm/your spindle speed. / is divided.