If it is stepper motor driven, then it could be that it is not using micro-stepping. The positioning accuracy given suggests this - you might expect higher resolution with micro-stepping. However, more to the point, stepper motors driven without micro-stepping, whether you need the resolution or not, can sound and feel like marbles in a cement mixer. Depending on the mechanical setup, this might be worse st slow speeds where there is little flywheel effect.

Only guessing - difficult to tell without hearing/seeing!