Thanks for the link, it is an impressive article and while I work in IT, the electronic may be a bit off my league (I just solder a bit around IoT devices and flash them).
I realized with your comment that I have never actually tested the plugs when they are next to each other (say, on an extender). I did not expect there would be a physical limitation below the advertized 1Gbps (I now all I got 400 Mbps I think but I am not at home to get that with a local iperf loop). If they cannot deliver close to that I would be pissed off :)
I have, again from memory, a 2x1Gbps tplink pair.
This said, my power cabling is sub par so this was the obvious reason for me for the lower bandwidth.