This is a little bit of a no brainer but it's possible to go from 85-90 in 2-3 hours without using any exp bugs/exploits (If you're like me who's too late to test out xp bugs and exploits that has already been nerfed then you might like this)
This requires a strong level 90 (preferably 555+ if you want a breeze) either a friend of yours or your extra account.
What you do is you powerlevel the sub 90 toon in stormstout brewery repeatedly, it took me 2 and a half hours to powerlevel my lvl 85 monk to 90 using my 573 warlock. I solo the entire dungeon within 6-7 minutes and when done just reque again, the lvl 85 that's being boosted do not even have to follow the booster, even if the toon stays near the entrance it will still get full xp from mobs and objectives.
The random people from the que would also appreciate it if you keep running with them since they too will be getting tons of free exp, if you don't have a friend willing to run you or an extra account, try to hire someone from trade chat to run you repeatedly.