do you mean literally THE fastest way? usually people have a group of people helping them, questing and burning through content. Hyjal is about 2 hours or so, and you're 82 when you leave there if you do all of it.
well quite often people pushing for realm firsts have a team of people on hand to help with questing, like kill quests etc. clearing out crap around quests, extra mobs that slow down progress, etc.
that, and some people even have a buddy IRL who quests for them while they take a break or sleep or so. im not sure on specifics but that's my interpretation of how some people pull it off