Yes remote storage is via the Dropbox extension.
Google Drive extension is also slow but I don’t normally use that, I was only seeing last week if it was also affected and it is.
Local server backups with no remote storage set up and not using any of the extensions is also slow.
I have the following issue, which only started happening last summer and hasn't been fixed yet:-
With no local backups and no remote storage set up, Backup Manager takes 7 to 11 seconds to load its main /admin/backup/ page. This used to be less than a second to load.
With local backups and/or...