aramis erak
Legend
Even in face to face...That's absolutely true on a session basis, but it doesn't necessarily explain why the progress of the campaign would be slow.
As you increase the handling time for any given action, the increase in players lowers face time each, and thus ability to move the plot from any given player's inputs, because the player of need either has been shut down by spotlight hogs, or has a smaller share than they would in a smaller group. The smaller the time share, the less one gets done. And with more vying for attention, everyone has fewer turns to get things done.
If everyone is good about large group dynamics, they know this, and will keep their actions laser focused upon the plot. If they aren't, you can wind up with lots of side action that's irrelevant to the plot but relevant to some subset of players, at the expense of the others.
Depending upon the game and the players, I've had this happen with anywhere from 5 to to 9 players. For 10-14, if you're not in LARP mode half the session, you need to be either very quick as a GM, or the players need to work through callers so the cognative load and communicative load is reduced, or run the game in miniatures wargame mode.
It's just the dynamics of player spotlight.
Ironically, I found 8p in FFG Star Wars did NOT cross out of my normal mode for 4-6, but 4 in the 90's ALIENS Adventure Game (by Leading Edge Games. Often noted as Phoenix Command Lite) need the shift to very controlled "actions only" mode simply due to the combination of combat heavy setting and crunchy as hell mechanics.