Hi, Michael.
The frequency the MP6500 switches at is dependent on how quickly the current in the motor windings reaches the current limit, which is dependent on lots of factors in your individual system like motor inductance, input voltage, and (of course) current limit. You might try adjusting the limit slightly or changing your input voltage to see if that helps.
It looks like there is a torque speed curve available on the product page for your motor that shows how much torque is reasonable to expect at various speeds, so you should check that you are not too close to that limit. After that, there are several things you might be able to change, like increasing the input voltage, to help you step faster. This post by Ben has more suggestions.
-Claire