Why is the streaming quality so poor on https://www.xfinity.com/stream/

I stream directly to my PC and streaming from the website (https://www.xfinity.com/stream/) is terrible. Constantly (and I mean constantly) buffers, looses bandwidth and quality and generally a terrible experience. Is there somewhere else I can stream live channels that I've pay for? Incidently, I stream video up to 4K from other sites all the time, and Xfinity Stream is the only place this happens. In fact, Amazon streams the local channels better.  I hope it is not intentional as I do not intend to rent the box.