Netgear Nighthawk X4S R7800 with OpenWrt 21.02.1 freshly installed
ISP line is 250/25. Wan port is 1G (auto).
Steps to reproduce: Laptop connected on LAN-side with 1G (auto): Ookla speedtest is 250/25. Laptop connected on LAN-side with 100FD: Ookla speedtest is only 30/25.
This performance degration only occurs, when the R7800 is routing and natting the traffic. "Switched only" traffic inside the LAN is not affected.
With stock ROM the download rate is as expected: Laptop connected on LAN-side with 100FD: Ookla speedtest is 80/25.