Update this is a Windows Defender issue not a database issue. https://spirhed.com/index.php/2017/07/06/windows-defender-issue-on-windows-server-2016-with-dpm/
This will be short, There seams to be a bug in the DPM 2016 database once it reaches a certain size or number of recovery points. We have 2 setup around the same time and both failed within 2 days of each other. The symptoms where backups just standing still not doing anything and if i open resource monitor of perfmonitor it would be slow and sluggish.
Now this happens after applying June patches during the weekend. But have reinstalled a server and restored the database with same effect. Only way to fix it was to reinstall DPM with a blank database. That solved the issue.
Microsoft escalation team is looking into this and we should be able to figure out what is wrong as i have backup of the DPM databases and can be replicated by restoring the DPM database on a new server.
Here is a screenshot of DPM and Perfmon with the issue.