Twitch user Fragnance claims to have gotten his demon hunter Sicklikeme (server Twisting Nether) to level 110 in five hours 28 minutes, and he streamed the whole thing. As a matter of fact, he's still streaming now and this time, his shadow priest leveled to 110.
Fragnance and partner Gingi spent weeks preparing for today in the WOW Legion beta. They knew the routes, the quests and their plan inside out. There are numerous archived videos on Fragnance's Twitch channel that show the beta preparation for the speed run. Apparently levelling in pairs is the way to go.