Buffering and Roku 3

I am not very tech savvy so please be gentle. My brother gave me access to his Plex server but most movies buffered. He said the Roku stick was probably the problem. I now have a Roku 3 wired to my modem and still get the buffering. If I pause the movie it will play for longer, but rarely get through an entire program without buffering. Any ideas? I have AT & T Uverse and the speed is fine for all other streaming services. Thanks for any help.

I would suggest it’s all to do with your and/or his internet speed!