Duplicity performance concern

On a fresh Ubuntu 18.04 system I decided to give Deja-Dup a go. It's just a GUI for Duplicity. About 850GB of data needed to be backed up. Source SSD was NVMe. Destination SSD was SATA. Initial (full) backup took about 7 hours.

The next day I did nothing more than check mail and install an application — which added ~230MB — before running Deja-Dup again.

Deja-Dup ran for ~39 minutes loading a single core at 35–70% for the entire duration.

Duplicity was invoked three times:

    The Scanning phase pegged a core at 100% for 18 minutes.
    The Backing Up phase pegged a core at 100% for 16 minutes.
    The Verifying phase pegged a core at 100% for 5 minutes.

This is on a new computer with plenty of RAM. The backup was not encrypted.

Now, I expect initial (full) backups to take a while, and that's fine. What I don't expect is for ~230MB of new data to take ~39 minutes to be backed up (and consume over a collective core-hour of CPU time).

I ran a manual backup after negligible changes (a few KB of new mail) and the time taken was the same (~39 minutes). Thus the time taken seems to have little to do with the amount of new data that needs to be backed up.

I ran iotop whilst that last backup was proceeding, and the Scanning phase Read 7.36GB, the Backup phase Read 868MB and Wrote 5.55MB, and the Verifying phase seemed to neither Read nor Write.

Is something wrong or broken? Should incremental backups of minor changes take that much time to perform? I was expecting something under 5 minutes, not ~39. Why is it taking so long?