I guess I've solved it. Did factory reset and re-configured everything manually. Latest result: 312/876Mbps (link). The only differences with my previous setup, I've noticed are: SPI firewall...

Long story short, I suspect my Archer C6 is throttling internet speed :/ To be clear: I'm not talking WiFi - all measumenets below are taken over Ethernet connection. So, my network setup is as...