The current version of the app is essentially an Objective-C shell with a Web browser inside. When it comes to speed, this is like putting the engine of a Smart Car in the body of a Ferrari.
Applications that are predominantly HTML5 render most of the components of an app as a Web page, pulling images and content from the Web directly into the application. Objective-C takes the opposite approach, taking full advantage of the hardware in the iPhone and then building most of the functionality directly into the application so it has to collect less information from the Web."
Facebook Plans to Speed Up its iPhone App - NYTimes.com