I think the OP is absolutely right. I have two iPhone 5s. One running iOS 7 and one on iOS 6.
Under iOS 7 I often start typing before the app is 'ready' in which case my first few key presses are missed. I also seem to be able to type faster than the keyboard can keep up with. Again, key presses are often missed. Of course, it's still in beta so perhaps the responsiveness will be improved.
Going back to iOS 6 on my other phone really does demonstrate the overall difference in responsiveness. Obviously newer software often requires faster hardware but, at the moment, by comparison, iOS 6 is a pleasure to use.