It's a hell of a filler game, and the short playtime allows for both multiple games per gathering or as a 'warmup' game while waiting for the rest of your group to arrive.
I think this observation depends on the group. New players who don't know the roles can slow this game down considerably. And even playing with experienced players, with a larger group this game can take up to ninety minutes or longer.
So although Citadels can play quickly as a "filler", I usually don't consider it as such.