A typical 5V, 1A stepper motor will have a 5Ω resistance. When the coil is energized, the current increases exponentially until it reaches its full value of 1A. As shown in the figure, this can be relatively slow. To drive the motor faster, the time for each step decreases. As the step time becomes shorter and shorter, it will reach a point where the step time is nearly the same as the time constant for the coil.