It is the F word that controls the speed of an axis when doing a move, so your axis is at X0 and if your code has G1X10F100 it will move the X axis to 10 at a rate of 100. What that 10 and 100 will be will depend whether you are using metric or imperial units, so it will either be 10mm @ 100mm/min or 10inch at 100 inch per mm.
This is all done in the G Code so it would be Vetric that you would set this up in as Mach will only do what the code is telling it to do, however you may have the FRO (Feed Rate Override) set low which would make things slow down, for example if it was set to 10% with the above code you would only move at 10 units/min instead of the 100.
Hood