When 29,952 light probes are selected, every UI operation takes an extra 90 seconds

This bug is further investigation of Case 562020, but using a different scene. This new scene has a lightmapped terrain, one spotlight, and a sphere which is lit by 29,952 light probes. Per feedback from graphics developers, this many light probes is not a realistic use case but seems like a somewhat reasonable number for stress testing.

I found that UI responsiveness bogged down when this large number of light probes were selected via Inspector. This could give users the impression that Unity is hung.

You can see the problem by doing the steps below, even if you skip steps 2 - 9. Steps 2 - 9 just demonstrate that the scene is set up properly and give baseline timing numbers for comparison against excessive timings in later steps.

1) Open scene: LotsOfLightProbes.unity

2) Lightmapping panel-> Clear
This takes no time.

3) Lightmapping panel-> Bake Scene
Baking the scene took roughly 1 minute on my Win7 PC.

4) Click the Play button.

5) Scene view-> Lightmap Display-> Use Lightmaps-> toggle on and off
Terrain in both Scene view and Game view change in response to Lightmap toggling, so the lightmaps seem to work okay.

6) Select Sphere in Hierarchy

7) Inspector-> Use Light Probes-> toggle on and off
In Game view lighting on sphere changes in response to toggles, so the light probes seem to work okay.

8) Click the Play button again to stop playing.

9) Project-> Assets-> LotsOfLightProbes-> select LightProbes
Inspector shows that there are 29,952 light probes.

10) Hierarchy-> select LightProbeObject

11) Inspector-> click Select All button
The Unity UI becomes unresponsive for roughly 2 minutes. Every few seconds, try clicking on menu bar items to verify this.

12) When the UI becomes interactive again, clear the light maps again.
This time it takes around 1.5 minutes.

13) Switch tabs in Lightmapping panel.
This takes around 1.5 minutes.

14) Bake scene. This time it takes 2.5 minutes.

15) Move another application window over Unity, then Click on the Unity window frame to bring it to the top again; this takes a while.

16) In Hierarchy select Sphere. Now normalcy returns. Clearing lightmaps and reordering application windows take no time. Baking the light maps takes 1 minute.