The Full Gameplay video posted is four players and an hour and a half long. Doesn't seem like they were playing super-fast to me.
Seems to me like the game wouldn't be significantly lengthened with more players since the number of Political Mandates played in a game doesn't change with the number of players.
Of course, more players do mean more potential 'follow' actions for some of the mandates, more potential participants in combats, and more possible discussion between the players which would lengthen play time by at least some amount. But I wouldn't think it would be linear.
Of course, I don't have no way to know for sure.
Keep in mind that the gameplay video is also edited. The most obvious cuts being the setup for each season.