I tried running the Space Blaster demo in Opera 11.51 on Windows 7 and it runs OK. It's a little choppy in places, but perfectly playable.
Perhaps you could upload some demos side-by-side for us to try ourselves?
A fair bit of performance work has been done on the games, so they should not be unnecessarily slow. I'm tempted to say Opera just need to make their browser faster, but if you say other HTML5 games run just fine then I'm curious to see them running side by side for myself.