Increase the ramp up stepper motor performance
A spinning stepper motor acts as an alternator; a voltage whose magnitude is proportional to RPM (called back-electromotive force or back-EMF) gets induced in the windings. The driver has to output more voltage the faster the motor spins to overcome this back-EMF, or the current will decrease.  

Torque is directly proportional to the current, so eventually the motor hits a maximum speed where the generated torque is less than the load torque and sync is lost:  



You may need to further increase the supply voltage to the driver to go faster. The motor is rated for up to 60 V.