To reproduce:

1. Create a new project and import the package (link in edit)
2. Drag Building_A into the scene (Assets > Prefabs > Environment > Backdrop_Buildings)
3. In the Lighting Settings window, make sure that either Realtime GI or Baked GI is enabled
4. Observe how the Editor starts lagging

Notes:

- Reproduced in 2018.3.0a6
- Couldn't test in any other version due to the prefabs breaking and throwing hundreds of errors
- Editor lags even when the prefab is selected in the Project panel
- Garbage collector CPU time eventually subsides, and so does GI Tick CPU time making the editor responsive again