Leveling from 85 to 90 on Timeless Isle - 10-25M xp per hour - solo

The best thing about this - It's soloable, and thus bound to get nerfed within a few days.
Should you team up with a lvl 90, you can expect to get about half the xp per kill, but the mobs will obviously die much faster and you get to kill more of them.

You can get to the island through the regular means: swimming/running with potion of water walking, getting a friend to fly you in or summon you.

What we will be killing are Spotted swarmers

Larvae with 80k hp that keeps together in packs

And windfeather chick

Birdlets at 160k who are also in packs.

There are a bunch of lesser mobs with less than 300k hp, almost everything on the island gives the same amount of xp, but these are the easiest to kill in numbers by far.
For references, the mobs give 16k xp at 87 and 19k at 88, and 22k at 89.
What is good with these, is that they are mostly left alone, so we can still level at a decent pace at active hours.

We'll focus on the larvae, they reside around Old Pi'Jiu:
at 87 you can expect maybe up to 2m xp from one full round around Old Pi'Jiu, killing swarmers, chicks and maybe even some ironfur herdlings if you got time to spare.
The swarmer has a spawn timer on about 6 minutes, this can easily put you on 20m an hour at 87.

You'll get some timeless coins and a little more than 1 gold from each pack, I don't believe they're skinable either. Maybe an occasional timeless item or a mote of harmony, but don't expect this to be a moneymaker. Potion of luck does not work either.

The only thing to watch out for is a stacking poison debuff and people with censer, needlessly to say, you are a free bloody coin and you might be better off with the debuff anyway.

If you're doing this solo, I suggest you get some nice gear from the auction house and dungeons, enchant it and flask up. That'll make it all so much smoother. Eventually you will also get Book of the ages, Dew of eternal Morning and singing crystal to ease the process further.
Also, expect downtime after being ganked and when running around to tag rares for their goodies.

If you run with a lvl 90, there's huge amounts of small turtles between the landings.

Should you stumble upon the Legends of the past you'll also find these turtles to be fun and easy to kill.

Edit: It is probably worth mentioning that I did this with a fairly spoiled brewmaster monk, so I had really decent aoe, survivability and healing, which meant fast kills and no downtime.