originaly, engineers disassembled their warmachine, and loaded them onto carts, to make sure that they would keep up with rest of the army. The machines were assembled agian, just before battle, and disassembled agian after. Don't know why siege was so slow in the vanilla version, but historically it wouldn't be that slow, but I think that the carts would have moved as fast as the average infantry man. Maybe you have to make the speed on the battle map of siege the same speed as infantry has, which makes everything more historically acurate.