It's a hidden blessing that Flash only uses a single core. On a single core, Flash will bring a computer to its knees, but a dual core processor will barely be affected at all. If Flash eventually supports multiple cores, then heaven help us.
Incidentally, it's actually braindead developers that are responsible for Flash being so slow. All they have to do is add a few wait states to their code and the problem disappears. I've actually seen a number of Flash games that were programmed properly, and will not use more than 3% of my CPU time, despite running at 60 FPS. True, it would be nice if Flash had a built-in way to throttle the performance of content, but... no HTML5 I'm aware of has a mechanism for doing this, either. Firefox certainly doesn't, despite having a Flash container process.
I'm always amused when people bash Flash for sucking up CPU time. If an HTML5/JS application goes into a badly-written tight loop waiting for input, it will max out the CPU, too... and will likely do so on all cores. Will a switch to HTML5 automatically fix code with tight loops? Of course not. Thus, I don't think it's reasonable to blame Flash for badly written apps, either. It's the responsibility of the client platform to manage process priority (the web browser is the OS of the future? Yeah, right. Even memory management poses a challenge to many modern web browsers!)
As for working badly, one thing I liked about Flash many years ago was that it was a paltry ~300K download that didn't require a full installer. It "just worked", when other things like Java weighed in at 15+ MB and were a PITA to install. Also, Flash content is almost always packaged into a single file, making content easy to download and archive for offline use (such as cartoons and games). Try that with HTML5 or Java. There is no official packaging format for HTML/CSS/JS, so archiving content consisting of hundreds of files with tons of cross-links is a lot more difficult today than in the HTML3 days. Even Java figured out that packaging was important with the JAR file format. With GZip compression becoming standard on most web servers these days, why can't HTML5 get an official packaging format? More complaining about it being against a free Internet or some other impractical crap?