Are you running in LPT mode or use an outside controller? The problem is the micro segmented code and the time element to get everything done smoothly.
IF the change of feedrate of .002 IPM in a move of .002" is a problem then you are using the wrong control software. Mach3 will never be able to handle that on its own ,MAYBE with an outside controller as the controller does all the timing work and can run MUCH faster internally than Mach3.
Just a thought, (;-)
(;-) TP