Once you are focused on a slide the change from one slide to another is small. For first focus the motor is quite slow. Moving by hand is quicker, there is a recent thread here with a method to quickly detach the z-motor Autofocus: motor fast detach system . This will make the motor less stable, that should not matter, although it might make autofocus less reliable.
The focal depth of high resolution lenses is very small, so the z-axis needs to have small steps. The motors are about 4000 steps per turn, and there are 4 turns per mm on the z-axis, or about 62nm per step.