Many Free-For-All games are lengthy slugfests. In a five player game, attacking one player leaves you vulnerable to four players. Attacking.
What games can you play with 5 Players in EDH? Wally D. introduces two of his playgroup's favorite five player commander games - Star Magic.
The advantage of the variant is that the game ends when just two or three tl;dr: What's your favorite 5 - player variant, emphasizing quickness.