"Recently there was a article about Firefox slowing down on videos due to the internal process that saves all open tabs every 10 seconds - you can increase this by going to about:config and increasing the timeout on browser.sessionstore.interval
That however would not fix the problem with Opera.
Personally, I have a good graphics card and fast CPU and see random problems, slow downs and lag in various flash videos. I think the latest few releases are buggy and need refining as I never had this problem in a older version on a slower machine.
I hope the first solution helps you as I know it has helped a lot of people - just not me!"