Looking at that article, the slowness doesn't come from having to download the entire RSS feed. As commenters there note, If-Modified-Since means you only need to download the feed when there are changes. So in your second paragraph, you have to ping the RSS feed 10 times, but you'd only need to download it once.
Rather, the slowness seems to come from the lack of an aggregation mechanism. If I want to follow 100 feeds (e.g. 100 Twitter users), I have to query each of those 100 feeds. Even though I can query them efficiently using If-Modified-Since, that's still slower than firing off one query to find out which of those 100 feeds have updates, and then querying only those which do.
Rather, the slowness seems to come from the lack of an aggregation mechanism. If I want to follow 100 feeds (e.g. 100 Twitter users), I have to query each of those 100 feeds. Even though I can query them efficiently using If-Modified-Since, that's still slower than firing off one query to find out which of those 100 feeds have updates, and then querying only those which do.