It's not one speed though. Mortal Savant immediately costs you 4 speed if you're moving from assassin. Even Swordmaster has 3 more speed, though Swordmaster is a poor endgame class too.
The inherent problem with Mortal Savant is hybrid classes are only potentially useful for giving utility spells to physical attackers, but utility spells are mostly faith based, and not enough people have spell lists and skills strengths that lend themselves to the class. There's just not much of a reason to want to use attack spells as a physical unit when bows and magical weapons can be used in any class and do the job better. Why would Felix ever want to use his bad spell list when he has a bow strength? Mortal Savant is just far worse for him than sniper or assassin. It's only characters like Manuela, Marianne, and Ingrid that are offered anything by Mortal Savant, and even they just have better classes to be in.