Prerequisites:
Download 'Free SpeedTree package from Asset Store - https://www.assetstore.unity3d.com/en/#!/content/29170.

Steps to repro:

1. Open Unity;
2. Import Free SpeedTree package;
3. Add new Terrain gameobject (Gameobject -> 3DObject -> Terrain);
4. In the Inspector go to 'Paint Tree' tab;
5. Click 'Edit Trees...' -> Add tree;
6. Select any tree from the list (in my case, 'Broadleaf_Desktop');
7. Click 'Add' button;
8. Place painting brush in the Scene view and click once on terrain to spawn some SpeedTrees on terrain.

Actual result:
The Editor hangs for 4-5 seconds. At this time Editor is completely unresponsive and doesn't react to any clicks.
Regression introduced in Unity 5.5.0a3.
See attached gif video.

Expected result:
Editor should not hang when painting SpeedTrees on terrain. The acceptable delay is somewhere about 500-1000ms (Unity 5.4).

Notes:
- Reproducible in 5.5.0a5, 5.5.0a3;
- Not reproducible in 5.5.0a2;
- Reproducible on both OSX 10.11.5, Windows 10 and Windows 8;
- Reproducible on laptops, haven't checked on more powerful desktop PCs;
- The hang is visible only in the new project when painting trees for the first time. After some time is passed, performance returns to normal state;
- Reproducible in both Realtime and Baked GI baking modes.