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.

Agrees but it was written by the people who created the framework so I'm sure they're much more aware of how to keep it performant.

Then again, you don't need to be the creators of Cocoa Touch to build high-performance iOS apps.

This. Anyone can make native fast for your average app, it takes a top-notch JS developer to make a similar web-app fast.