Changing speed with the controller has a curve, but I'm not sure what it is. Off-zero rpm torque is pretty good with that type motor though so I doubt you'll have a lot of trouble with it. If you need significantly lower speed but with good torque you want a mechanical reducer of some sort. Pullies, gears, whatever. That will multiply torque at the same ratio it divides RPM at. Larger lathes have whats called a back gear for high power / low speed work. Mine will run down to something like 50 rpm at the spindle and the motor runs at 1750 rpm. Its slow but it will happily rip the arm right out of your body if you get wound up in that thing and that flat leather drive belt is not going to slip before it happens.