Rams should move only slightly slower than infantry, after all, they are presumably being pushed by infantry (otherwise, their just rolling of their own accord). Even if the ram weighs a lot, ten infantry should be able to make it go at least as fast as a single infantryman.
An idea for balancing (and realism) is that rams and other siege machines should be stationary until their are people garrisoned in them. Siege towers and seige machines with people as part of the actor would be the only exceptions (and ships, which presumably have a rowing crew onboard).