Whenever AI Traffic is enabled (Default in-game traffic), it causes huge FPS loss in the Scenery Editor. What happens is lots of aircraft spawn on top of each other in the middle of a runway, thus causing performance loss. It's really easy to reproduce and happens 100% of the time.
Make sure Traffic is enabled and at 100 density, open an airport project and it will happen. It can cause drops as low as 5 fps.