Fast, Streaming Updates: This is kind of a big deal, but it's also an easy need to address. Most RSS usage isn't the kind where subscribers are refreshing their feeds every minute or so. Ultimately you have to hit a separate server for every person you follow.
But in the wake of the Google Reader shutdown, we now have several RSS aggregator services (Feedly, Feedbin, etc) all set to poll any RSS feeds we might subscribe to. And because most of these services mimic Google Reader's API, there are a number of apps that sync with those services.
This means that we already have an ecosystem of light-weight RSS apps that can rely on third-party servers to do the heavy lifting of updating feeds. It's still decentralized, but it could in principle be sped up to support near-realtime updates without too much difficulty.