I have no experience with your hardware so someone else will need to answer those questions.
With a settling time of 20 ms the highest frequency at which the motor could respond fully and settle before moving again would be 25 Hz. Each half cycle would take 20 ms so the total period would be 40 ms => 25 Hz. If you could achieve a sinusoidal excitation, you would likely be seeing both phase shifts and reduced amplitude above approximately 25 Hz.
Lynn
