The millipede is a segmented arthropod that has 2 pairs of legs per segment. This is because they are twice as awesome as centipedes as they only have one pair. Seriously, though, even with the extra legs per segment, millipedes aren’t known for being exceptionally fast. They are actually quite slow little things if you actually watch one for a while. Due to them being scavengers, feeding mostly on decaying organic matter, they don’t have much of a need for quickness (I didn’t want to say speed there because I’m tired of that phrase).
I wouldn’t consider them to be much of a pest unless you are an avid gardener constantly tending to seedlings – something that they like to destroy.