I have registered this issue:
Improve first-visit experience · Issue #5591 · duplicati/duplicati · GitHub
I think it reflects your experience, but let me know if I misunderstood.
Absolutely right! The built-in .Net zip is much faster than SharpCompress, but only supports Deflate+Store algorithms. In case any one else experiences this, the solution is to request SharpCompress:
--zip-compression-library=SharpCompress
For most cases, there will be an automatic fallback to SharpCompress, but in this case the fallback mechanism is not triggered as .NET was able to open and read the headers without problems.