Mobile Safari performance is absolutely worse than a native app. That is why I'd like to know how the HTML5 apps running within a webkit frame in the FB app will perform. If the answer is similar to a native app it would allow HTML5 apps to 'code once and deploy everywhere' within FB's apps. Which would be f%$&ing amazing.
I suspect however the answer is that they'll perform simliar to Mobile Safari, which as others have mentioned has some pretty severe limitations for all but the simplest HTML5 apps.