Facebook for iPhone is, to put it gently, a piece of crap. It’s painfully slow.
According to a recent report, the app is set to get much faster. The reason it’s currently so slow is because it’s basically just a web browser inside an Objective-C shell. Content is constantly being pulled from the internet, this causes a lag. The good news is that they re going to dump all that and give the application a major overhaul. This should make it much, much faster.
It will be built in Objective-C says the report, and most of the functionality will be built directly in to the App rather than using the website. This will make things flow much more smoothly.