Flash and Youtube are not exactly the same thing and not apple to apple for comparison. As pointed out, most major sites are changing over to check for Flash 10, even if some of the content is not 10. Sites that worked and played content no longer work and ask for the newest Flash player. Seems to be enough posts to suggest there may be a problem with (not Youtube) Flash performance since the upgrade, but again- moot, since most sites will soon not work anyways. Well, the ads will still work, of course

One way to get around the Flash 10 gate keeper issue at sites for content that is 9 or below is to use Google video to search and play videos that launch directly from the search result. I have noticed this gets around the issue, to some degree.