I wrote about a problem with USN before:
On one Win 2016 server, I had to downgrade from 2.0.5.1 to 2.0.4.23 because the backup took about 15 hours. I back up about 800,000 small files on the server with a total size of 250 GB. After the downgrade the backup takes only 30-60 minutes, the changed files are always only a few thousand.
I also tried to split one backup job into several smaller jobs, but that didn’t help, the total backup time would still be around 15 hours, because the newer Duplicati skipped USN almost always. But sometimes USN has been used, but I don’t know why.
Now I have the new version only on 3 computers, elsewhere I stayed on the old beta.