Reproduction Steps:
1- Open the reproduction project (case_863912.zip).
2- Open profiler and start profiling the editor.
3- Measure time taken by saving a scene.
3.1- Open Scenes/Transform/Transform_10000.unity scene.
3.2- Click on "Menu bar -> Test -> Test/Measure Save Current Scene". This will mark scene as dirty.
3.3- Save scene.
3.4- See the time taken by save operation in profiler.
4- Measure time taken by entering playmode.
4.1- Click on play button.
4.2- See the time taken by EnterPlaymode in profiler.
5- Repeat from step 3 with other scenes to test with different amount of game objects.

Expected Result: Entering playmode and saving scene scale linearly or close to linear.
Actual Result: Both operations scale exponentially.

Reproduced with: 5.6.0b1, 5.5.0p3, 5.4.3f1