Way back during my original RAF, Disc Priests leveled faster than Elemental Shaman...
That was during the late Burning Crusade era.

Almost every mob, needed a little more than 5x Lightning Bolt... so it was that, plus a second cast.
Meanwhile the Priests could 5x Smite, and that was enough to one-shot quest mobs in the world.
Smite did (and maybe does) a decent amount of damage; and with the whole Atonement Healing thing... even better.