Streaming Video Incredibly laggy, can't even watch

I was surprised to find out that comcast has some free shows on "watch TV" even though I don't subcribe to their cable service, sadly, they are completely unwatchable. I tried to stream Season 2 Episode 3 of Under the Dome, but the video will start lagging and stuttering making it un watchable. I've even lowered the stream quality to "low". I've tried a couple times and it never works.

Today, no more than 5 minutes later after trying Comcast again, I load up the same episode on Amazon Prime Video (I watch the free version that has ads so same service as comcast except I pay $0 for it) and it runs flawlessly and in great quality. Both videos are played on a Windows 7 PC, both in chrome, both on my 50+ MB/sec connection. Two different sites.

Comcast runs so much more poorly, what's up with that?