Slow ethernet on RT-AC68U
Upgrading my home internet connection from 100 to 600Mbit didn't make as much of a difference as I expected. speedtest.net reported barely over 300Mbit on ethernet or WiFi. Connecting directly through the modem (some Huawei) gave the expected speeds. Turns out, the Asus RT-AC68U has a setting in Advanced Settings / LAN: "NAT Acceleration" was disabled and setting it to "Auto" (the only other option) enabled "CTF (Cut Through Forwarding)". This was all that was needed to get the same speeds through the RT-AC68U as directly from the modem.