Experiencing WAN speed issues since day one with Archer C6

Computer directly on modem → 920Mbps
Computer via C6 (ethernet) → max 420Mbps

WiFi speeds on 5Ghz were ok, as expected for wireless.

2.4Ghz was a joke, but I don't mind since only some old smart-home devices use it.

When I tested

- No other devices were connected to the modem/router whatsoever. 

- I tested the two cat5e I used with direct connection to the modem and both gave around 900Mbps

- I used exactly the same speed-test endpoint, provided by the internet provider.

- Latest firmware (at point of creating post) was installed.

Unfortunately it seems C6 is not powerful enough to handle it.