I have been using Mach3 with a DMC1750 controller and 3 axis servo motors and encoders for years. It works fine, and speed has not been an issue. The only problem I have had is with ver 4.6 of the Galil-ST plugin, which does not update the DRO correctly. It seems to operate open loop rather than closed loop the way Steve Murphy says it should. Ver 4.4 seems to work OK, but may have other issues. The way to tell if it is updating correctly is to power the motors down while Mach3 is open, and move one or more motors manually. The DRO should update the move if it is reading the encoders. You can also turn power off to the motors and send a G-code command from Mach3, e.g. G1 X1 F20. The DRO should not change if it is working correctly. If it changes, Mach3 thinks you have a stepper motor and an open loop system, which is the case with ver 4.6.