If you read about FriendFeed's Simple Update Protocol (http://blog.friendfeed.com/2008/08/simple-update-protocol-fe...), you'll find some good arguments about why RSS slow. The crux of the problem is that to get any updates on a feed, you have to request and download the entire RSS feed (which is filled with summary text and is of nontrivial size).

Consider the case of a real-time news aggregator. A news source doesn't update very frequently (say once every 5 minutes), but you want to provide updates super fast (say within 30 seconds). That means on average you have to scrape the entire RSS feed 10 times for a single update. On the other hand, FB/Twitter APIs give you the flexibility to do things like "download items after this id," which hog much less bandwidth.