I would guess that reducing the number of civs would slightly reduce the wait time between turns, but that the main costs scale with the total area under AI control -- thus, you need to reduce the map size to improve your time-between-turns.
I usually play on Small maps for that reason. Unfortunately I don't think you could make a plausible Earthlike map in such small confines.
I usually play on Small maps for that reason. Unfortunately I don't think you could make a plausible Earthlike map in such small confines.