Arthur uses YouTube and he says that it's so compressed, it's absolutely unwatchable. Leo says that when he's streaming video, it's largely dependent on his bandwidth. The less bandwidth he has, the lower his resolution is going to be. He can adjust the quality he's getting in the settings, however, but at the end of the day, he may need to just get more bandwidth.
The issue also may be that he's not getting a consistent bandwidth streaming. Arthur should check out SpeedTest.net. That will show him what he's consistently getting to stream. Then there's network congestion to consider. Is he sharing his bandwidth with others who are streaming? Mobile devices, computers, and TVs are all trying to get a piece of the pie.
The other issue could be that the source material is loaded in lower resolution. If it's an older program, it could have been uploaded in standard definition. There's not much he can do about that. Also, if he has wired his TV's streaming source into the router, he would get far more consistent streaming than if he were attaching it via Wi-Fi.