Well Sam did lot of speed improvements lately, and the Theme I talk about (Amber) is efficient on the Pi. Also keep in mind the Pi is $50 compared to at least $200 for a complete Ion setup, right?
Yeah but it's not the UI that's laggy. It's mostly the library updates. I've got a Raspberry that takes 15 minutes to do a library scan, while my Ion takes about a minute, maybe 2. It depends on your use case of course, but when you use sickbeard, you end up doing library updates almost daily.
You don't need to do a full scan each time. If you enable SickeBeard XBMC notification it will only add the new Show without updating the whole library...