If the move you are making is all written on one line - e.g. G0 X10 Y20 Z2, then the computer works out the relative speeds of the axis so they all arrive together - as though it were a cutting move, so therefore in the above, if you take the rapid as the axis with the longest travel, Y, X will move at 1.2 Y speed and Z will move at 1/10 Y speed.
If you put the moves on separate lines, they will move at whatever the rapid speed is for that axis.