What I’d suggest is either a test backup of some area that can run without filters (to see if it scans fast) or avoid the backup part completely while (presumably) keeping the filters in The TEST-FILTERS command.
Exclude list for windows talked about the slowdown from that list (though your slowdown might be worse)
Filter groups code is extremely slow #3395 talked about issues. You’re not using OS-specific filters, but a large number of do-it-yourself filters might see the same issue. I am not a filter expert, and definitely not a performance expert regarding the Duplicati filters, so first thing to do is to check if your slowness is filters.
I’m not sure how much parallel work is done. If none, your I7 might max out one logical core, at 12% CPU. Adding in the overhead might raise it to 30%. CPU use running test-filters might be interesting to see.