The g-code is put out by Sheetcam, so if you are having to edit the g-code in Mach3 to change the cut speed, it sounds like the Sheetcam Post Processor is where the problem is. You could look at the G-code file as it comes out of Sheetcam with a text editor and see what it looks like there just to verify my hypothesis
Without knowing what your Post processor or equipment is, that's the best I can come up with.
Precision Plasma gantry
CommandCNC Linux controller w/Feather Touch & PN200 hand controller
HT-45 plasma cutter