If the thumbnail file is ever erased, such as what one might do when trying to troubleshoot an issue with blank thumbnails that never resolve, the game (CAS in this case) will be much slower for a while until the thumbnail file rebuilds itself. It should catch up eventually, though that can take several game sessions. I can see where selecting every CAS item in turn might speed up a rebuild process a bit or one that never accounted for everything until then. It's the same with the other two thumbnail files there, except those are for downloads and Build/Buy catalog items rather than CAS.