Air is the only ride where you go slower after merge points - crafts are sent every 3 minutes from one station if they're on 3 trains (that's time waiting for the craft to arrive, plus time waiting for the craft ahead to leave lifthill). Obviously before merge you have both stations, so you have 2 crafts every 3 minutes. Obviously Fastrack can slow it down, however it shouldn't do so too much (Fastrack for Air is 20% of capacity, and rarely sells out nowadays).