[35.4.1] LAN Tx performace is getting lower when HDMI is plug out

Hi,
Probably the developer kit is somewhere physically broken. If it is in warranty, you can consider do RMA process:
Jetson FAQ | NVIDIA Developer

The latest test we do is to connect two Xavier NX developer kits in Jetpack 5.1.2, and do iperf3 test. While plugging/unplugging HDMI port, performance drops for a short while and goes back to normal. If you see different behavior, it seems possible the board has minor physical damage.