My iPhone 4 seems to handle iOS7 pretty well. I've found that although the UI *looks* slow, the responsiveness is actually not that bad - for example, when entering my PIN on the lock screen, under iOS 5 and 6 sometimes there would be a bit of lag and it wouldn't register all the keypresses if I tapped the numbers faster than the phone was ready for. With iOS 7, there's even more lag in the visual and auditory feedback when pressing the buttons, but it registers all the keypresses correctly despite the lag, so even though it looks worse it works better.
And of course it's not just slow - everything looks worse. The new Windows 8-inspired theme looks stupid anyway, but on the iPhone 4 all the transparency effects are disabled, so it looks even worse, but it functions. There are some nice features and enhancements. I really haven't had any significant problems.