jQuery Mobile is certainly one the most (if not The most) optimized platform to build on right now. It offers 'depth' & 'breadth' in almost all the core areas one might expect. I have been building on it since it was in Alpha and have personally experienced the team roll out fixes upon fixes, which leave the competition in the dust.
Discussions of what is the best platform are futile. The answer to that debate is always: it depends.
For example, looking at mobile web page transitions, jQuery Mobile is way behind the competition in terms of optimization. The slide transitions in Sencha Touch 2 are way smoother than jQuery Mobile. I recently had lunch with one of the Sencha Touch developers and learned that they optimized this by not using CSS3 transitions at all and instead use a different approach with much better results.
To pick the best mobile web framework for you, you really need to evaluate what you're looking to get out of it. Then go with the one that has optimized those things which you care about most.
I've used both jQuery Mobile and Sench Touch and ultimately I decided I value a small framework codebase and full control of the framework most. Thus, I use a handrolled solution and it's been a great decision for me.
Any details on the approach they used? In my experience, it's impossible to beat css3 transitions on a platform that hardware-accerelates them (i.e., only iOS as of this writing)
He said they use horizontal scrolling to slide the page in from the side. I haven't yet dug into it to see how they're doing it, so unfortunately I don't have any more details, but you can download Sencha Touch 2 and check it out.
I guess the difference comes from using -webkit-translate3d which kicks in hardware acceleration instead of -webkit-translateX or simple -webkit-transition.
Kudos!