Although it even rings true for desktop browsers not being able to perform as optimally in regards to some HTML5 features work, Apple can and could do more to improve HTML5 support but won't for the foreseeable future until it works out how it can control and monetise HTML5 (which it can't and never will).
People are quick to point out Facebook ditched web and went native because of HTML5 limitations which I believe is a lie. It's been rumoured that Facebook is going to be deeply embedded into iOS 6 like Twitter got in iOS 5 and one of the requirements would obviously be to have a native application.
The Facebook app I believe was slow because of one key issue: 37 requests, 491kb. That was the problem, of course that many requests and large size is going to be slow for users on congested 3G networks.
Case in point of just how well a web app can run is LinkedIn's iOS app. It works the same way as the Facebook app used too. It has a Node.JS backend and uses a web wrapper to load it in, they even wrote up a blog post about how they squeezed every ounce of performance out of the web UIView and I think it works well.
I'm not one to believe in conspiracy theories without solid facts, but in my opinion the move to native by Facebook was a strategic one on Apple's end. Apple knew Facebook were the poster-child for HTML5 web applications that could run without relying on the almighty iPhone operating system, so throw them an integration bone and ask them to go native in hopes that others do the same and ditch HTML5 as their first choice for an app and many people will follow.