So I settled on borg. I use it for offsite backup. I did restores of individual files as well of whole snapshots after broken hard drives. There even was a time I used it with WSL to backup my parents data.
I suppose the latency of the remote server was too high.
Except that I tested them and they didn't. I hadn't "lost locally cached metadata". And it didn't just scan all the source files again, it re-transferred them over the network. All 4TB of it! That's why it took so long. (At that time the sever was still local, because I wanted to make the initial run not over the Internet. So there wasn't even added latency.)
I'm sure it was a bug, maybe even in combination with OpenBSD, which moste likely is fixed now. As I said it was years ago. I compared and was more happy with borg. Which stood the test of time and saved my ass multiple times.
`restic backup -v -r <location>/<repo_name> <source_dir>`
first `cd` to the parent directory of the `source_dir` to avoid too many nested directories in the repository and conflicts in the mounting point path (so that the file change detection of restic works when mounting the drive to be backed up at another point)
The performance issues you experienced may have been resolved.