This completly relies on you, having a geared lvl 90 character who can boost you.
We all know the method in Stormstout Brewery, where you clear up untill the first boss, leave the dungeon and go again. Now, doing this with a lvl 90, gives you about 2-3% of your lvl a run. And takes about 1½-2 minutes to pull off.
Now given you have a lvl 90 char who can boost you, as soon as you hit lvl 86-87, enter Mogu'shan Palace, clear the entire first room (Not the hallway), and you'll get about 2½ % of your lvl. This took us 35 Seconds on average to do.
Optionally, you can take that lvl 90 and a single lvl 86-87, and clear the entire instance(2 man - Enter manually), and you will recieve about 16-17% of your entire lvl, killing all the mobs. [U]This took ME 9½ Minutes. And corresponds to 80% of your lvl / Hour. (Every Mob was giving me 20-40k Exp, with bosses from 170k-210k).[/U]
Enjoy it untill it gets nerfed!