When selecting the MapMagic root object it slows down the editor if a lot of gameobjects are generated/spawned. Please move all generated gameobjects to a new root to avoid this.
Usually Unity slows down the performance a bit when selected an object with many child objects. However this should not be THAT noticeable. How many objects do you have and what is the frame rate?
I spawned 10 forests on one tile with increased density. Maybe several 1000s trees. It is very laggy in the scene view. Maybe because all the objects are highlighted when selected...