I finally got it installed on an iPhone 4. I haven't tested it a lot bit the lags seem less noticeable now. It's still not fast by any means but it's a noticeable improvement. From what I can remember, it's still not as fast as iOS 6.1.3, but it's much closer. The iPhone 4 with iOS 7.1 is what iOS 7 on the iPhone 4 should have been, but better late than never.
Note though that iOS 7.0.3 after a fresh reboot usually felt OK too. I guess as memory filled up, it bogged down. Not sure if 7.1 just doesn't bog down as much, or else the bogging down isn't as noticeable because it's faster to begin with. Probably the latter though or maybe both.
Still, even with 7.1, I think upgrading to it is only a no-brainer (for non-jailbreakers) on the 4S and up. With the iPhone 4, it's more of a decision. For me, the decision is to go with the more modern OS, and it's much easier now with the performance of 7.1, but not everyone would agree because of the performance issue (and/or because of the aesthetics).
Analogous to the iPhone 4S, my iPad 2 remains responsive, but it was already responsive on iOS 7.0.
I've had that issue in 7.0, haven't upgraded to 7.1 yet.
Which issue? The faded backlight setting at screen-on?