The disk was largely idle most of the time, although every few hours there’s a 45 minute process that pegs the disk (waiting 6 hours before it starts again).
I was watching Process Monitor, there was only CPU usage from Duplicati and virtually no disk or network access. Disk queue overall was sitting at 0 - 0.1 for the majority of the time.
The subdirectory I was restoring into was virtually empty, after about 30 hours a bunch of 0 byte files appeared and then nothing again for the hours until I decided to build a VM just to handle the restore (and completed the entire restore on this VM before killing off Duplicati on the original machine).
The parsing described in the code explains why it was trying to work with local blocks at all, thanks, but even so I don’t really comprehend why nothing happened, Duplicati had time to scan the entire drive several times over if it wanted to do so.
I don’t believe the machine was particularly overloaded in any fashion both because I keep half an eye on it all the time, and because after I set up Duplicati on the VM I got Restic installed, configured and completed over 50% of a backup (uploading directly to B2) before my VM finished the restore, all the while the original Duplicati install was still chewing away without a single actual file being restored.