Windows VSS/Snapshot policy logic

As i understand, with snapshot-policy=auto backup uses vss for accessing all files.
I got a big performance issue with this option. Is it possible to use vss only on locked files?

I don’t believe Duplicati has that capability. But I’m curious as to why you see such a performance issue. On my system the time to create a snapshot is negligible, and reading the snapshot is the same speed as reading the normal disk.

Can you tell what is slow: actually creating the snapshot or…?