I ran a backup of my backup home directory, to a USB drive with full-disk encryption. Here's the result:

04h00m13s 172739 files; 211.24 GiB up (15.01 MiB/s) /home/liw                  

Not a speed daemon, but adequate.

Backing up over the network, or using encryption in Obnam (rather than the device mapper) makes things slower, but I'll optimze those next. The slowness is because Obnam is currently entirely sequential, so anything that adds a frequent delay (e.g., network round trip times, or running gpg) has a pretty big impact on the overall speed. But that's fixable.