Correct, SS has zero protection against slows. Immobilizes are the only thing of concern on this front - Combat Jumping provides this, and is inherently faster than hovering via bunny-hopping. Almost every good build takes Combat Jumping for said Immobilize protection - whereas Flight as a pool has to spend a third power pick to acquire the same protection.
Super Speed + Combat Jumping is two powers, to explain. Fly + Hover + EM is three. This is setting aside that you have to spend more enhancement slots on capping flight speed (for both Hover and Fly) than you do on capping run speed or jump speed. I hope this explanation helps! 🙂
Edit: I'd like to add, as well, that Hasten is essentially one of the strongest power picks in the game. It just flat-out makes more sense to take Super Speed/Combat Jumping vs taking three powers to have equivalent Immob Protection, only to end up requiring more investment to be fast as well.