I watch a Youtube video for instance. I see high cpu usage in the system monitor around 80 to 90%.
At first I didn't know what was happening. In Firefox I opened the Task Manager, and VDH was shown as the top consumer of resources while watching a Youtube video. No download was active, and the cpu is consistently high while watching the video long after page load. I disabled VDH, and boom, cpu usage dropped to around 10%.
I'm on Linux if that matters. And of course using Firefox as I have already mentioned.
It doesn't seem to happen with every video I watch on Youtube. But when it happens on some video it continues to do high cpu usage on that video page, and does it even if I restart Firefox.
BTW there was one question about something similar, but I don't know if it applies to my particular case.