Hi,
concurrency-max-threads already has a default value of 0 and should use the existing hardware capabilities for good performance. I’ve changed this to a fixed number of threads, but I see degradation instead of improvement.
concurrency-block-hashers and concurrency-compressors have a default value of 2. Increasing those to 4 doesn’t seem to make a difference (or at least not a noticeable or noteworthy difference).
I’m not saying that improvements are not possible, but at first glance I don’t succeed. I suspect (very) small improvements can be found, when I would test in a very fine grained way with many, many combinations of different parameter values and in-depth analysis of resource usage, but I don’t think it’s worthwhile for me to do this right now (I’ve already tested multiple value combinations and I don’t see any “opening”…).
However, thanks a lot for your help and insights, @drwtsn32! I’ve decided to use Duplicati for certain backup types (i.e. towards OneDrive for Business), so I’m certainly in (although partly) 
Greetz,
Pedro