Ensuring a game of Arkham Horror runs quickly even with new players
It may sound silly, but in these cases (especially with so many players) the "we came here for kicking monster asses, not for pleasure" attitude should help immediately. Focus is a key word. Make sure those aren't playing at the moment are planning for the next round, it could help the focus effort.

In addition, I'd try to hint or even say straight up that they need to get themselves up with a strategy; it could be some kind of role separation - who closes the gates? who clears the monsters? Who gets the weapons and distributes them? - it could be geography separation, etc. But it'll keep the players focused on what they should do every turn instead of re-strategizing everything every time a monster pops up.