Refer to the article Plex behaving slow and make sure to run these tasks on a regular basis
Restart the plex server
Run "refresh metadata" on the libraries that are missing metadata as shown in the image above
I have tried everything and nothing works!If step 1, 2 and 3 above in combination didnt work, it is most likely a corrupt database - a re-deploy of the server would be required.
Please open a ticket to get further assistance!