I don't know for sure, but for me personally and how I play, the more characters I have to lug around the slower I go. If I were to skip all collection quests, maybe that would be a diffetent story, but my OCD makes me have to complete whatever I start.

The fastest I ever went was when I dualboxed a Priest and a Mage to level 9 or so in 2 hrs doing the Dwarf starter zone. So maybe that's the perfect balance of speeding up kills, but not slowing quests down.