Animation editor is taking up 100% of one CPU core

I have this problem on multiple pc's with the same project.
I've also tried it in Unity 3.5 , in the hope it was just a bug, but unfortunately, it still happens.

I've attached a project, basically if you follow these steps, you will get the problem:

- open it
- load test scene
- click on background object in Hierarchy
- open Animation window (ctrl 6)
- try to do something to enter edit mode: for example go to 120 keyframe

Note:
It is slow even after selecting a GameObject at the bottom of the hierarchy (such as "yourLabel" ) and filtering in the AnimationView with the Show:Animated only.