Expected speed of practically unchanged incremental backups

Yes, though the files will still be deduplicated.

Restic uses a prior (“parent”) snapshot to optimize the backup process. If a file has the same path, size, and mtime as in the parent snapshot, the file is assumed to have been unchanged. If this is not the case, the file contents are read, chunked, and each chunk is stored in the repository if it doesn’t already exist in the repository.

The parent snapshot is automatically selected as the most recent snapshot that has exactly the same hostname and path set as the current backup. If there is no prior backup with the same hostname and path set then no parent snapshot is used and all files must have their contents scanned. However, the parent snapshot can be manually overridden with the --parent flag.

I’d need to see the list of snapshots and know which snapshot was created in the second example to be able to tell you if a parent snapshot was used.