But they do not mechanically affect the movement rate; at least, not directly. If you move at a walk (and your GM does not feel that you need to make a Moving Maneuver roll), then they would not affect the rate.
If, however, you wanted to run, then that is going to require a Moving Maneuver, and that -30 penalty would apply to the maneuver, so you will move less distance.
Requiring a roll for paces higher than walking was common in RM2, but Companion IV (p. 8, 'Movement Pace Anomalies') explained why this is a problem: namely, because requiring a roll means that choosing a faster pace often results in the character moving less of a distance.
So, according to the Rules As Written, the penalty really would not affect the character's pace unless the character were attempting something the GM decided required a Moving Maneuver roll.