I was noticing this morning that their "I don't want to see this" feature seems to be stored client side. For a second while the page is loading I'll see an article that immediately poofs away because the client side js reads the setting and hides the article. Every single time this morning that I refreshed the page, I saw a few articles poof away. They really want me to see the tabloid crap I marked as don't want to…
Facebook could be processing this client side instead of serving the content already filtered so you could un-hide content almost instantly. Either that or ease of development.