Welcome to the forum @renato3
The symptoms here are kind of mixed. Gaps in uploads can occur if preparation falls behind, because uploaded files fill towards a size limit (default 50 MB, but Options can set Remote volume size higher).
The long duration and low system load suggest it’s not a preparation limit, but let’s isolate issue some.
You could look in your temporary folder at files starting with dup-. How many 50 MB files are queued?
You can also look at About → Show log → Retry to watch Duplicati trying to parallel-upload those files.
For how long? Does that look like 50 MB? You can watch uploads at About → Show → Log → Retry.
Log files at profiling level can give an exact per-file upload speed, but it’s a bit cumbersome to set up.
How remote? Duplicati uses SSH.NET for transfers, and it has a known issue that can make it slow.
Improve SFTP performance on medium/high latency connections #866
SFTP issue with high bandwidth high latency connections is the discussion from the Duplicati forum.
Maybe try the same test using Export As Command-line to get the URL, modify it to use empty folder, Duplicati.CommandLine.BackendTool.exe upload. Or try Duplicati.CommandLine.BackendTester.exe.
Commandline in GUI can run these, or you can try whatever native command line you’re familiar with.