I've got 4-boxes. Up to level 28 I was doing questing, but the levels were coming in so slowly (3 hours per level roughly), and the collection quests so unbelievably annoying that I switched to using my 70 hunter to run the 3 28's through instances at high speed for xp. Only looting after the final boss is dead. They are now pulling in about one level per hour, and are currently level 35/36 by runs through SM Cathedral. Granted, the fourth account isn't leveling any more, but that account already has my 70 hunter on it, so no biggie.

In the past I've found it faster to level an alt on a second account by just instance runs as well. Those were always just on /follow though. Now accounts 2-4 are actually kicking off instant spells to help the main, so the overall DPS is being helped out.