You might need to adjust log levels. Profiling will show SQL queries – potentially lots of them though.
If it’s in an SQL query, that’s largely single-threaded (thus 25% load on 4 cores), but also look at I/O.
Resource Monitor possibly gives a nice view. Exotic tools like Process Monitor are also available but
Task Manager Processes tab disk activity might be easy way. I think it’s looking to avoid downloads:
Duplicati will attempt to use data from source files to minimize the amount of downloaded data. Use this option to skip this optimization and only use remote data.
says maybe you don’t need any download optimization, but too late for that in the middle of the restore.
After it figures out which blocks it needs from destination, it should download dblocks to distribute them.
I’m hoping you’re not at default 100 KB blocksize, because that’s too small for 2.1 TB (but too late now).