[88keyz]

I consistently see 320-330 Mbps down on a Fire TV Stick 4K with a Ugreen USB 3.0 ethernet adapter on my gigabit network connection, the 2nd gen Fire TV Cube should see similar if not better results.  Other USB 3.0 network adapters with the same ASIX AX88179 chipset should see similar performance.  Here are a few that are sold on Amazon.com, regional availability may differ.

https://www.amazon.com/dp/B00M77HMU0/
https://www.amazon.com/dp/B00MYTSN18/
https://www.amazon.com/dp/B00AQM8586/
https://www.amazon.com/dp/B07N2ZHFY9/
https://www.amazon.com/dp/B00FFJ0RKE/
https://www.amazon.com/dp/B00LLUEJFU/