Why do Ustream and Livestream run my computer so hard?

I don’t understand what makes them such system hogs.

I can watch Youtube videos in HD with no problems.

I can watch HD movies with no problems.

But trying to watch a stream gets my computer burning and sometimes crashes it from overheating.

It doesn’t make any sense. A stream is downloading and watching video at the same time. And the video quality isn’t even that good. What else is happening that makes my computer go overtime?