The best way to speed up a more-player game is to allow more than one player to play at a time. This is easiest when you are playing two teams, but can be done in a free-for-all-game as well.
In a four player game where player are seated:
W- - -E
The players N and S could play at the same time, while the players W and E also play at the same time. Just be careful to move all chargers before other movement and make some house rules on models which simultaneously charge each other.
In a five player game we usually play star. Players are seated as follows:
G - - - - - - U
R - - - B
The players W and B start the game. As soon as W and B have finished, the next players clockwise play, which are U and R. After that, players G and B play. We have the rule that the player seated left of a player looks along with him as he is playing (checking any dice rolls, etc.) while the unoccupied player is the referee that turn.
This causes some chaos, but speeds up the game tremendously. After 2 games we wouldn’t have it any other way.
Players with 3 or more than 5 players are less than desirable and require special scenario’s to play speedy and enjoyable. Most often we appoint an arbiter who thinks of a special scenario to help things a little.