I wouldn't worry too much about leveling fast. unless you are a twink, heck even if you are, concentrate on making money. do all the money quests and the levels will come along nicely.
Ratmen right outside of GLudio is where i ground all my pre 20 characters. And the bugbears in tha tlittle stone circle are great fo rnukers since they arent social or aggro.
I would argue against leveling in the starter dungeons. There are non-dungeon mobs around each starter village that give better xp and are easier to kill. For example, on TI the golems at the western/northern side of the island give upwards of 705 xp while the dungeon mobs only give ~300-400 xp.
My twinked human fighter made level 15-20 on these golems in 5 buff sets (unbuffed except for haste/alacrity pots) and didn't use a single heal potion.