Not super fast, but best is More Help for Innoruuk. Farm belts from the three orc camps in The Commonlands. With a mount you can run between all three camps and keep them clear around the time they respawn. Belts drop pretty frequently, orcs respawn 2.5 minutes from death. You will also earn Common Residents, Ashen Order, Guardian of the Vale, and Knights of Truth from the orc slaughter. The hand in lowers Frogloks of Guk, which can be repaired by killing in Lower Guk.
Belts provide +5 Dark Ones faction per. They do not stack, are not lore, can be handed in by 4s. Can speed up travel to/from Grobb and The Commonlands by using the LDON Magi.