Hi Phoenix,
Welcome to the forum.
Mach will run any or all the axis at the speed you set in motor tuning section for the axis in question (up to the point the motor stalls,of course).
Just for reference, my Z axis is set to run slower than the others, mainly because of the dead weight it carries and the finer pitch leadscrew fitted. Unless you are planning on mulitpass, deep 2.5D work a slower Z should not cause any real increase in program execution times.
Tweakie.