Do you experience similar issues when playing HTML5 video on YouTube?
https://www.youtube.com/watch?v=7gu8q_fJeVU&html5=true
If you right-click in YouTube and choose Stats for Nerds, can you take a screenshot of what it looks like when it's performing poorly? That would be interesting.
I'm curious about whether or not disabling Hardware Acceleration has any effect on this.