After finally getting fibre to the home my ISO download speeds went up dramatically compared to my old DSL connection and now the slowest part of the NZB download process was the unpacking part. I just had NZBGet downloading and unpacking onto my root filesystem (two SSDs in a ZFS mirror) so I figured I could move them to separate drives to see if it would improve performance. And the results were quite amazing.
I went out and bought 2 1TB Samsung 860 PRO drives then put them into my server and formatting them with ext4 (with journaling off), I swapped the downloading and completed directories for NZBGet.
At first I couldn't believe it but after testing a few more times I confirmed the results. So if you find your unpacking process is slow then add some dedicated drives for the downloading and unpacking directories.