Facebook comes up a lot in these discussions and it's important to provide some context. Facebook's HTML5-first strategy pre-dated the launch of the original iPhone. In the first native mobile app they were simply rendering the mobile facebook site in a UIWebView. There are so many better ways to do HTML5 and HTML5/native hybrid apps these days. Check out the Basecamp app, for instance. It's every bit as good as Facebook's mobile app but it makes heavy use of HTML5.
This demo from sencha touch has always seemed dubious to me. I remember having coded extremely basic mobile web app with this framework and having had layency issues from almost day one.
My guess is that what they are doing with their framework to get to this result is pretty far from a normal use.
I think if you develop from the ground up to be a mobile optimized web app you can achieve good performance.
I recently built this as a side project using Bootstrap and it feels similar to an app (or faster) on iOS 'saved to my homepage': http://moneytrackr.ca/
Then later I found out about the Ratchet framework, so I think the potential is there.
