I'm not sure if it is the maximum acceleration limits.
I tested PWM input, and that has around 20ms response.
My test setup: MCU Sends pwm signal and turns on an LED, and I film it using my iPad's 240fps camera (which gives me a 4ms time resolution). The motor does not move at all for 4 frames, and then starts accelerating slowly. 4 frames means 20.8 ms of delay.
Since the PWM sets the current, even with the acceleration limits, I should have seen some sort of movement.