I did some non-scientific performance tests.
Mac Mini running El Capitan,
Destination: Seagate Backup Plus Hub attached via USB3 port. Blackmagic Speed test reports read/write speed of 160 MB/s.
Internal MacMini disk reports read/write speed of 85 MB/s.
Backup set consists of 4 large movie files (total of 5.6 GB) and 100 photos (total of 230 MB).
Backup1: Seagate, Encrypted, Upload volume 50 MB – backup speed 9.4 MB/s
Backup2: Seagate, Encrypted, Upload volume 250 MB – backup speed 9.9 MB/s
Backup3: Seagate, Encrypted, Upload volume 500 MB – backup speed 9.4 MB/s
Backup4: Seagate, not encrypted, Upload volume 50 MB – backup speed 13.3 MB/s
Backup5: Seagate, Encrypted, Upload volume 50 MB, disable streaming transfer – backup speed 9.6 MB/s
Backup6: Seagate, Encrypted, Upload volume 50 MB, disable streaming transfer, use move-for-put – backup speed 9.1 MB/s
Backup7: internal MacMini drive, Encrypted, Upload volume 50 MB – backup speed 9.4 MB/s
Based on these limited tests, the only significant speed improvement can be achieved by turning off encryption. The USB-attached drive is as fast as the internal one.
However, looking at my real life large backups, which are in the 200-400 GB range, I can achieve only about 2.4 MB/s in a best case scenario. So Duplicati seems to struggle with anything that large. Also, Duplicati started behaving very weird with this large backups, like sometimes getting stuck, running very slowly, or overheating the CPU due to 800% usage.
Further, even with less than 1% CPU usage of mono per Activity Monitor, the MacMini runs very sluggish during backup operation, opening windows or apps takes a long time and other software is slowed down. I wonder whether the code is just not optimized for Macs.