If you have a VERY high polygon model, it will chew up FPS. For example, I was once testing out a 50,000 polygon model with SpawnFlags 2, and it wouldn't load the map. An error about too many brushes.
Spawnflags 2 make every seeable side to the model a clip brush. That is all right for low detail models, but do not do it for very high detail/poly models.