I disabled file compression and it seems to be a bit better, even though still fairly slow.
Other problem, but still related to i/o slowness, I tried to run the same rdiff-backup (with file compression off) on a USB drive (which is formatted with btrfs with compression enabled) and at the second/third run, it crashed due to some weird i/o error [read only device].
I suspect the high i/o activity made the "slow" USB disk crash. It would probably work if it was an SSD.
It's a shame that rdiff-backup is so i/o hungry but I suppose that's the way it is for now.
Otherwise I really like the tool!
Bob - yes, lots of files under /mnt/vms/docker/ have more than one hard link.
I might try your suggestion (--no-compare-inode) if I run into trouble again.
What problems does rdiff-backup have with hard linked content?
As far as I know hard links should be supported...