In most contest people using stepper motors struggle to make a confortable run due to the slow speed of the stepper motor and a poor torque . Unipolar motors have lesser power than their bi-polar counterparts but then there are ways to make your unipolar stepper work better than what it does now.
First You can have a look at the stepper motor control mechanism here
so in that post we’ve seen that the pulses are given in the order as below.
Now this will do a good job and give you a smooth running robot. But another good alternative to run your unipolar motor with greater torque and speed is to give the pulse the other way as below
This sequence will do the magic for you. Bu the robot will have a bit rough run compared to the single line sequence given previously.The two line drive will show you how fast your steppers can run with an appreciable torque.