What a great thread and thanks to everyone, especially Tupsi for persevering. This issue had me pulling my hair out for weeks now trying to find the root cause of my network being half speed in one direction for my OMV NAS running on a Lenovo M910q Tiny. I ruled out anything physical on my new Cat6 LAN setup early on, got into all sorts of network troubleshooting, tried so many iperf setups, performance tuning, etc. etc.
Only when I googled for half gigabit speed, this thread showed up. I tried the fix above to disable TCP segmentation offloading, and it works!