Update for those affected.
I've started another topic in the family section of the forums where you can read more about the bug's journey. - https://feedback.photoshop.com/photoshop_family/topics/cloud-libraries-broken-since-last-update-photoshop-19-1-6?rfm=1&topic_submit=true
Thanks to the Engineering Department of Adobe Creative Cloud Libraries, we've been able to debug and figure out the problem.
Back in beginning of September Cloud Libraries have switched their method of thumbnail generation from server side to client side. This unearthed a very old Adobe bug where some files would be saved with gigantic amounts of trash metadata, bloating the size of files that should be 40kb to over 30mb (There are several topics for this on the forums). If you try and open those metabloat files in chrome (which cloud libraries use in their panel), it would take about 30+ seconds to open. This made the panel lag very badly and search queries taking up minutes at a time as a result.
Now all the thumbnails that the Cloud Libraries panel have generated could have been affected by the metabloat, mind that only some files are affected.
You can browse the thumbnails in the C:\Users\[username]\AppData\Roaming\Adobe\Creative Cloud Libraries\LIBS\[hash]_AdobeID\creative_cloud\renditions folder. Search for overly large files to see if you're affected.
Until Adobe implements a fix for the thumbnail generator, you can use optimization tools to make the thumbnails smaller (Like PNGGauntlet). If you have a lot of libraries, you can use a small script file we've made for our team to optimize all the files at once:
Linky: File “tools.zip”
Drag your renditions (Contains png files) and dcx (contains psd and some png files) folders over the removemetadata_all.bat file and leave it working. If you have a lot of files this can take a while. I had 11058 files across 56 libraries with unoptimized thumbnail size of 7.5GB that was optimized to 564 MB
So far it's unknown if this temp fix is sustainable as we don't know how often thumbnails are regenerated. New items in libraries will need to be checked for metabloat as well to retain consistency. But at least the libraries are usable once again. Hurray.
dcx