Its pretty slow..but im ok with it because his range is so large and he has so much dmg and he has an escape and he has his ultimate. If there is a champ that should get 325 ms, its azir.
He has four ways to survive an engage if his flash and zhonyas is up. He has his dash. His ulti. Zhonyas. And flash. If he had good movespeed as well, he would get a nerf to his dmg, and i dont think you want that.
The adcs like draven and jhin who dont have the same range or disengage actually need the ms to dodge skillshots. They dont have as much time to react as azir does at his range. At least thats my opinion.