Here are my suggestions for speeding up the base game of Doom.
Marine players all play at once or as near simualtaneous as possible.
However before they all start making their moves, the Invader player is given the chance to examine any interrupting cards in his hand and advise the players wether he may or will not be playing any of them. The Invader player must be given some lenience by the Marine players to play these cards to thier best and even have the Marines take back a part of their turn as required.
Next, when it comes time to lay out a room the Invader player should read out the items to be placed and the Marine players do all of the searching through the boxes ( and baggies ). This speeds up the building up of the rooms.
The next point is possibly going to cause the most arguement. Limit the invader to activating no more than 8 alien-monsters per turn.
Of course, this makes the game just a little easier for the marines, but it does make the game more enjoyable, and keeps everyone busy, more of the time.