Yes, it's because the Facebook app sucks. Pretty sure the lag is due to FB being busy gathering data about you and the link you chose, bundling it together and passing it on to whoever they're selling it to, or something along those lines, before pulling in the appropriate page and packaging it for viewing within the FB app. Sounds paranoid, but FB is massively successful company for one reason: they know how to skim data and sell it very, very well.