Released 6.99.2 with the following fixes:
1) Fix the performance problem with the Manage X dialogs in very large libraries
2) Fix editing comments in the content server broken
3) Fix editing notes via the raw HTML view not having the changes saved until you switch back to the normal view
4) Fix updating metadata in some badly made PDF files with lots of garbage very slow.
5) Fix an issue with errors while using the Manage X dialogs and metadata for books being backed up in the background at the same time
https://download.calibre-ebook.com/betas/