There's no rule that says you need to run your motors as fast as they can spin. You can just set your velocity in Motor tuning to whatever you want your rapids to be.
That's all you really need to do, if your happy with your resolution.
Are you running direct drive?
Do you have enough resolution? You could always fabricate a belt reduction, which is cheaper than gearboxes.
Reducing the power supply voltage will slow it down, and the motors will run cooler. But I don't really see the point in doing that. If it were me, I'd do a 2:1 or 3:1 belt reduction to increase resolution.