I think your feed speed is set low in your Cam program or whatever you are using to generate the G code. Look for feed speed in there. You can manually edit G code in Mach3 as well, to increase the feed rate. You can also check the feedrate override box in the General Config. I haven't tried that, but think it would give you max machine speeds during a G code run. This could be very bad if you are actually cutting, but useful to dry run the code on the machine with a higher Z axis setting.