Actually I just asked someone who updates iPhone 4 to iOS 6.1.2. After updating, its ok for common operations, but if want to run large programs, of course it is very slow, but acceptable still. It is not that serious like cant stand the speed.
Fluency of iOS 6 and iOS 5 is almost the same, I think.
The new version is not that smooth like old one, but it has more functions, of course.
Fluency of iOS 6 and iOS 5 is almost the same, I think.