The number of GameObjects and Components in a Scene have a significant impact on the time the editor needs to open and save a scene, as well as entering and leaving play-mode.
I did various tests a while ago, the results can be seen here:
The conclusion is that the Unity editor does not scale with complex scenes. Where “complex” refers to the number of Objects in such scene.
Unity confirmed the problem, but nothing happened since then:
While I was trying to find a workaround to manage really complex scenes, I tried to split that scene into several hundreds smaller scenes and load/unload those smaller scenes on demand in the editor.
This actually did work, but I ran into another issue with Unity. That is the editor does not support async-loading:
The result was few milliseconds freezes while navigating the scene view and a new scene needed to be loaded. This caused the scene camera to go completely crazy for example, not usable in production.
I doubt they’re going to fix either of these issues anytime soon, if ever at all.