I have only checked the performance of the 2.5G interface with linux 6.12.10 - not the 1G interface. Could you check if the issue remains if you use linux 6.6.74 ? (install with 'dpkg -i linux*'; replace the dtb with the one attached; execute 'update-initramfs -u'; reboot)   Edit: The 2.5G interface needs the hardware fix in order to operate reliably in mode 2500baseT/Full too: during ethernet handshaking 1G speeds could be established transiently. I think that this might have happened