Long time lurker,
I've found an interesting thing about quest mobs recently. It seems there is a "Minimum spawn" where if there are less than a specified number of mobs active, then the spawns become instant. I'm not sure if this is a temporary feature or a perm one, but if 3-4 people got together and sat on different respawns, they could force them all to become instant! (Mote farming FTW)
If your solo like me : Just find a quest mob where TONS of people are, and sit on a respawn. By you taking 1/4th of the respawns, you actually CAUSE a shortage and can create an instant respawn scenario even if its not one when you got there.
-Lance
84 Mobs:
Horde - When you get a quest to go kill Faeries who are stealing supplies (I apologize this isnt super specific) The Faeries have only 4 spawn points and are commonly instant respawn.
Kun-Lai: Level 87
On the Island on the bottom right hand corner there are lvl 87 mobs.
You can find "Enraged Priest" and those are a quest mob (Kill 14) - There are a few spots where I found to be INSTANT respawn during heavy play times, and those drop motes of harmony at a rate of 30-40 per hour, while giving 14k exp per kill (at lvl 87) This equates to a level in about 2.5 hours, which is on par or slightly better than questing.