This makes me wonder... does anyone offer an iperf-based speedtest service on the Internet?
With it in the way I get ~15-20Gb/s
$ iperf3 -l 1M --window 64M -P10 -c speedtest.init7.net
..
[SUM] 0.00-1.00 sec 1.87 GBytes 16.0 Gbits/sec 181406
$ iperf3 -R -l 1M --window 64M -P10 -c speedtest.init7.net
..
[SUM] 0.00-1.00 sec 2.29 GBytes 19.6 Gbits/sec