It's more a blow to UIWebView than anything... it's slower than native OS calls, but it doesn't have to be that way forever.
The UIWebView is slower than Mobile Safari because it's missing the Nitro Javascript engine. (Apparently, for security reasons.) I suspect the Facebook HTML app would have run decently had Nitro been in UIWebViews.
You're right, it doesn't have to be that way forever.
Hopefully, Nitro has been brought to UIWebViews in iOS6.
If not, well darn.
Sorry, this is really off topic: I usually prefer just simple HTML interfaces to apps when I am just reading content. I have just stopped following TechCrunch links on my iPad because they always redirect to a nag-page to install their iPad app. I tried to follow a link yesterday and something new happened: I was never allowed to see the page because the redirects back to the nag=page were continuous. What assholes. I know that it is their content and they can do with it what they want, but this seems like a crazy business decision.
edit: I don't use Facebook's app either.
I think HTML/Javascript will always be slower and clumsier than a native API, mostly because the former is a creature of committee-think, and the latter less encumbered by the need to placate a bunch of different interests. That's not to say that the former, which has the truly great compensatory virtue of being open, will never be a good choice for application programming.
Even though I have developed some javascript code, I always feels that my javascript code is full of hacks and needs to hack along the way.