[phiwum]

If your NPCs are okay with enslaving elementals thus, then they should turn their attention to transportation next. An air elemental can speed up a nautical voyage -- or a land voyage, if one mounts a sail on a wagon. A water elemental in "steam mode" might do even more work, if there's a handy and quite talented mechanician available.

But earth elementals seem not to be well-suited for such clever abuse. Lucky them.