It almost killed me to get to level 10 with my pally and 4 mages. Part of that was getting things setup and ironing some kinks out of my setup. I was doing quests but became frustrated with the availability of them and the logistics so I ended up an instance grinder. Much more fun and my gear is quickly turning all blue.

But like the others said. XP is split up. It would probably be better to focus on doing as many quests as you can.