Hmm, sounds CPU bottlenecked, yes. This might not be so bad if we can get LZ4 compression implemented for Duplicati.
I believe there is an other thread about preseeding data from an other machine on the forum here Possible to relink an existing backup store (for pre-seeding purposes)
That might be helpful. Otherwise the options might be either just waiting for it to finish or tweaking the --zip-compression-level to see if that helps performance at the cost of compression ratio. Note that the compression level has to be set before any data has been uploaded because there isn’t a built in way to change it later.