You might want to check out TwimGo - a qtwrt based Twitter client available at substanceofcode.com. Not slow at all.

The RSS reader is pretty quick also - the UI is responsive and loading and parsing of the RSS feeds is quick as well. I might have to strip off some of the CSS3 effects to keep the scrolling smooth but other than that you can do pretty heavy things with QtWRT.

When I release it anyone can take the code and easily tweak it as they wish.