Reproduction steps:
1. Open the attached project.

2. Open and play 'TestAssetBundles' scene.
3. Check 'Use Dropdowns' checkbox.
4. Select anything from the dropdown list.
5. Click ' Download Bundle'.
6. Wait for 5 seconds.
7. Click 'Clear GameObjects'
8. Click 'Clear Cache'.
9. Stop the scene.
10. Check Editor CPU usage.
- It will stay low (about 3% for me).

9. Open and play 'TestSceneChange' scene.
10. Click 'Load and store Asset Bundle' a few times.
11. Click 'Clear Dictionary' and 'Clear Cache'
5. Stop the scene.
6. Check Editor CPU usage.
- It will stay at about 100%-400%.

Reproduced with:
iOS, Android, macOS Editor, macOS Standalone, Windows Standalone
2017.1.0f1, 2017.3.0b2

Couldn't reproduce with:
Windows Editor