My favorite feature of the iPhone app is how fast it is to open a feed from the list. Tap, and usually the list of articles shows up in (what feels like) less than a second.
Folders, on the other hand, seem to be disproportionately slower. I know that a folder has to fetch multiple feeds, instead of just one, so it would be somewhat slower but it still bugs me to the point that usually I will just open each individual feed because I feel that I can get in and out of those faster than waiting for the folder to load, scroll, wait for more loading, etc.
Have you considered loading the folder contents async and letting articles from each feed in the folder *pop in* as they are downloaded? This might help improve the perceived speed of the folders and let the user start reading their articles right away while the rest downloads.
The only other idea I have right now is to start prefetching the feeds (just the info needed for the article list screen) when the app launches. You could even put in some basic heuristics to determine which feeds with unread items are opened first most commonly. (I have pretty predictable patterns such as opening my Netflix RSS feed first if it has unread items, or conversely, never opening X blog first if there are multiple with unread items.)
If nothing else I think updating the “Loading…” indicator to something more helpful for longer waits would be good. Perhaps something as simple as “Loading 1 of 25…” and incremening the numbers as a progress indicator so I don’t get bored waiting for the “Loading…” text to disappear.
Once again, this is only a problem for me on the folder views. Individual feeds open very fast and I love it.