Not exactly a whole lot is slow about the movements of their "slow" units other than the fact that they don't have transports (assuming you discount tomb spyders), and pariahs and immortals have to hoof it (again, immortals only if you discount tomb spyders).
Every other unit only moves slow while fighting. They all have a teleport on an individual cooldown, and flayed ones have deep striking without a cooldown. (Deep strike, recall, deep strike)
You basically don't run into a single necron mobility problem unless you're playing on gigantimous map and the other player's abusing half his vehicle pop full of transports to keep running away as he attacks your oversized base.