I've noticed that if I use SwiftKey, the keyboard some times takes a while to come up. But once it does, SwiftKey is very fast. I've experienced some lag in the app switcher a few times.
I was having issues with the Camera app freezing up for lengthy periods of time in iOS 7.1.2, and similar things are happening here.
So far, everything else feels about the same as iOS 7.1.
I would rather have the new features, and am willing to accept some slowdowns and screen space cramping to get more life out of this device. I don't mind the slowdowns as long as once things finish loading it is fast. It's not annoying like keyboard lag in early versions of iOS 2.0, or iOS 4.0 on a 3G which completely destroyed it's usability. I've never used an iPhone 4 w/ iOS 7.x, so I don't know how that compares, but the way people complain about it, I imagine it's not as bad as that.