I can't speak for Windows RT and Windows 8 because I haven't had a chance to use either extensively but I have used Windows Phone 7(.5). In the past six months

I can't speak for Windows RT and Windows 8 because I haven't had a chance to use either extensively but I have used Windows Phone 7(.5). In the past six months I've gone from a Samsung Focus to a HTC One X to a iPhone 5 and back to a HTC One X. Despite running on seemingly antiquated hardware, the Focus and Windows Phone 7.5 was just as fast and reponsive, if not more so than the other two devices.