I suspect this is NOT what’s going on for you, but I did want to mention that more recent versions (I think AFTER your 220.127.116.11) included a change that ended up making backups run more slowly. If it does turn out this is the cause for you, a fix is already pending.
Oh, and I just realized I never answered your question about the retention policy stuff from April 6.
The retention policy code only kicks in if you are using a “Backup retention” (on step 5 “Options” of your job edit) other than “Keep all backups”.
While an aggressive retention policy combined with lots of daily source changes could increase perceived backup times (it’s really just increasing the retention cleanup step that happens AFTER the actual backup is completed) I suspect that’s not the case for you.
@Pectojin might have a thought on how to verify, but I really think the database size is the cause of the performance issue for you. I know @kenkendk has an update coming to address the size as a few other users have asked about it, so hopefully that will be available soon.