I dualbox 5 shaman, and everytime I think about questing, I pull up my guide, go to the places I need to be at the level I'm at. Gather all the quest, and then I start trying to do them and realize how much it sucks.
Gathering are really bad for multiboxer, even one person alone would take a while to gather the stuff, the probably is not really the gathering part, just the drop % and the amount of limited mobs to kill. Hey give me a 1000 of everything that drop my quest items to kill in a row and it won't be a problem, just whenever I clear one place you need to wait for it to respawn it's really bad.

I'm 45 right now, and it's getting slower in instances, mostly because ZF and Maraudon are not very good for exp. IMO. They are wide instances and takes a while to clear it, unlike SM Cathedral, but eventually I'll get through it and go to ST in a few levels.