Is your ROKU a little box, or is is also a stick? I have only used a ROKU box or an Apple TV box for streaming. I do not think the sticks have enough memory, hence the buffering issues. Also, I connect my ROKU and Apple TV to my router via LAN cables. I mainly use the Apple TV for streaming because I have a lot of iTunes movies that I watch on it.
It’s a box that has Wi-Fi so it doesn’t store movies, etc. It connects you so that you can stream them. It holds it’s own software and information and that’s all from what I can tell. The Firestick is a stick. The Roku works much better.