Expected speed of practically unchanged incremental backups

That’s definitely slow. Backup of HDD (NTFS) to a rest-server in the same network:

using parent snapshot 1754f7f6
creating VSS snapshot for [d:\]
successfully created snapshot for [d:\]

Files:           6 new,     5 changed, 198687 unmodified
Dirs:            1 new,    18 changed, 21837 unmodified
Added to the repo: 6.081 MiB

processed 198698 files, 579.080 GiB in 2:32
snapshot 16b7e2c0 saved

Note that your log misses the information about a parent snapshot. If restic is unable to detect a parent snapshot (because of changed paths) it will re-read all files in order to make sure it doesn’t miss any changes. Obviously that’s a lot more time consuming.