They specifically targeted people who boost to slow them down.
https://us.battle.net/forums/en/wow/topic/20761996118

MiRai and Svpernova have been leveling teams, some videos here:
https://www.dual-boxing.com/threads/...Ganis-US-Horde

I too would like to know the best method, which changes for every level range it seems like. I am guessing you'll mostly be questing, run each dungeon once for the quests, then back to more questing.