If you feel adventurous, I would sugggest trying out the new canary builds. They are no longer based on Mono, and we increased the default block size to 1MiB, which both contribute to faster execution.
Happy to hear you have found a solution. I was testing performance of Duplicati, and can see a steep drop after a certain size in the database, and I hope to fix that.
That is actually also the direction Duplicati is going. The latest versions are without dependencies, and also have a .zip package. You can see the canary builds here if you like.
And the CLI interface for Duplicati is actually only 2 commands: backup and restore, if you prefer not having the UI.