When you say the system is calm, something is keeping the disk 100% busy (if I translate correctly):
Figuring out why an idle system is running the disk as hard as it will go would be a good first step…
For this you can try sorting columns (there are lines not visible now in screenshot) and researching.
I am not hugely familiar with Resource Monitor, but the information you may need is likely in there…
Interestingly, after you start Duplicati the disk is slightly less busy (per blue box) but queue is longer:
It looks like you have a VSS snapshot created, and Duplicati is looking at it. Obviously this uses disk.
Previously mentioned –usn-policy will sometimes save scanning through source files for file changes, however any files that are found to be changed must still be read to figure out what changed in them.
Beyond this, Duplicati uses temporary files for its processing, which can upset people with SSDs who worry about write cycles. RAM disk can solve that. You spoke briefly on these alternatives in this post.
The latest screenshots support my theory that the system is disk-limited, but more analysis is needed.
As previously stated, I leave this to you, and that’s even more true with 100% busy without Duplicati…