Reproduction steps:
1. Open the project "repro.zip"
2. Open the profiler, and turn on "editor profiling"
3. Open the scene "_8Artois_HIGHMEM_ASSETBUNDLE"
Notice that GISceneManager.Update() is taking ~90% of processor time.
Notice that GI isn't even turned on.
If the project is closed after the lag stops, and started again, the lag will resume.

After several minutes, the performance will return to normal. The performance hit seems to depend on the number of objects in the scene.

I've attached a screenshot of the profiler screen. [Actually, FogBugz doesn't let me attach anything ATM (503 error), so if whoever reads this needs those files, I can send them over via slack.]