Something strange happened. IDK. I used your factory image (because my GIGA was using vendor firmware) and it showed poor performance, then i sysupgraded it to generic openwrt with the same performance. Now i sysupgraded again to your image and now I get my 2 gigabits. And yes, i had HW offloading enabled.
[ ID][Role] Interval Transfer Bitrate Retr
[ 5][TX-C] 0.00-10.00 sec 1.09 GBytes 937 Mbits/sec 8 sender
[ 5][TX-C] 0.00-10.00 sec 1.09 GBytes 935 Mbits/sec receiver
[ 7][RX-C] 0.00-10.00 sec 1.06 GBytes 914 Mbits/sec 0 sender
[ 7][RX-C] 0.00-10.00 sec 1.06 GBytes 911 Mbits/sec receiver
Also in your factory image wan had type "switch port" and in your sysupgrade it has type "Ethernet adapter". Perhaps there's something wrong with factory IDK.