Reproduction steps (Compare between Unity 5.5 and 5.6(GIFs attached)):
1. Open the attached "Hierarchy" project.
2. Open and play the "1" scene.
3. Make sure Stats window is open in the game view.
- You should have around 15 or less fps.
4. Add some more game objects if it shows more than ~15 fps.
5. Expand the "Particle System" game object in the hierarchy.
6. Notice how the expansion animation behaves.
- Instant expansion in Unity 5.5
- Sluggish expansion in Unity 5.6

Regression introduced in:
5.6

Reproduced in:
5.6.0f2, 2017.1.0a5

Not reproducible in:
5.5.2p4