The u-boot snip:
Comphy-0: PEX0 2.5 Gbps
^^^^^^^^
means the SERDES line 0 is used for PCI Express. RGMII is dedicated bus (not SERDES) with 1Gbps RX and 1Gbps TX. Hence, I think, EspressoBin can’t handle more than 1Gbps throughput by hardware design (without help of additional NIC connected to the PCI-e slot).
If you consider routing between two 1Gb ethernet networks connected to two 1Gb ethernet ports (for example wan and lan0 in the EspressoBin case), maximum throughput is 2Gbps — just because each port can receive 1Gbps at maximum (2Gbps in sum) and there is no way how to get more data into the router. How did you calculate the value 4Gbps?
