stepper motor constant speed under load?
A stepper motor, as the name says, moves in steps. Each step is a fixed angular movement and the number of steps per second defines the speed. It will always move at the same speed no matter what load you put on it until the load is so big that it simply can't move. At that stage it will starts to skip steps or outright stop if the load remains too high. You will hear a clicking noise when that happens.