It sounds like you need to do some motor tuning. Go to Config/motor tuning, choose an axis, and press the up/down arrow keys to move the motor. Change the acceleration and velocity settings until the motor seems to move fast and smooth, sounding zippy rather than agonized. Then derate your settings by 25% or so to give yourself a margin of safety and remember to "save axis settings" before moving onto the next. The curve should look like a mesa, rather than a volcano.