The "Backup Manager" link in Plesk takes too much time to open.
Loading the "Current Backup Tasks" tab is fast, but "Server repository", or "FTP repository" are slow.
The issue is caused by big amount of backups in server repository. When Plesk creates a backup, each object (like client, domain or reseller) is described in an XML file. Backup repository has hierarchical structure, so when the big server backup is listed, it will iterate through all the child XML files. If it is too many of them, it can take time. Server performance also matters significantly in this case.
Check the following parameters which are related to backup files scanning:
HDD fragmentation. If it over 30% then run Windows defragmentation utility.
Numerous of Backups was created.
The backups amount and their size.
If there are a lot of backups, it will take some time for all of them to visualize in "Server repository".