I gave up researching optimum exp for lowbies.
By the time I figure out what's optimum, I could have earned 2 levels AoE grinding.

What's optimum is:
Quest without P-level until 10.
Share RFC quests, and Boost with a 70 in RFC.
Grind until 13.5, and port the lowbies to TB, and UC to turn in the shared RFC quest.

Run to SFK and Boost grind from 14 to 20.
Run to SM and Boost grind from 20 to 40; Cathedral and Armory only within 12.5 minutes. Skip Armory if you are clearing Cath at 12 minutes.

At 40, Summon to Uldaman, and Bind in Kargath.
Boost grind 40 to 55 in BRD, doing some instance quest.

55-58 can be done in BRD, or do WPL until Argent Dawn Friendly, and turn in the EPL Lights Hope Chapel quests (Bone chips, Dark iron scraps, etc).

Or 55-60 Grind some Ramparts/Blood Furnace. Great factions before doing any Hellfire Quests.