Is there a speed limitation when using USB ethernet adapters? I have gigabit internet, it would really stuck to be limited because of Nintendo's stupid decisions.
You'll have to use a USB 3 to Ethernet adapter to get full speeds. The highest theoretical speed over USB 2 is 480 Mbps.
You're better off just getting a USB 2 to Ethernet adapter because there is no video game that will ever need all that bandwidth. You only need the the adapter for potential latency reasons, not speed reasons.