The speeds are via the status panel towards the top of the web UI. They’re also based on an initial backup of about 12.5GB. A separate 550GB backup took about three hours.
CPU is 3700X, so 8 cores / 16 threads.
The SSD was indeed a speedup attempt. Regardless of destination, the source was an NVME SSD. Whether the destination is set to a second NVME SSD in the system or a USB 3 HDD, speeds were identical.
I’ve concluded that it had to do with the sheer number of tiny temporary files that were included in the backup.
I feel like this can be marked as resolved.