Let me answer your question, for I've been testing iOS 7 beta 6 the latest week in my old iPhone 4: Yes, the performance is pretty decent, and though the GM is near, this is still a beta, keep that in mind.
I'm surprised of my iPhone 4 being capable of running iOS 7, because the GPU of the A4 is very similar to the one in the 3GS, and has a display with four times number of pixels. Impressive.
Lets make it clear: obviously, moving through the entire system, multitasking carrousel animation, lists like the Settings one,
are not smooth as silk. BUT, if you don't mind minor frame downs (already present on iOS 6), the system is absolutely usable. I was thinking of selling my iPhone 4 in order to buy the new iPhone 5C, but seeing the performance in my old iPhone 4, sincerely, I don't know what to do.
I repeat: Compared with an 4S or an iPhone 5, it is not as smooth, and it's slow opening apps too, (but this was that way on iOS 6 too, is iPhone's monocore A4 fault), but the beta 6 works -surprisingly- pretty decently.
The first days, the battery of my iPhone 4 on iOS 7 beta 6 was terrible. I mean, terribly bad. Maybe half of a day. But that's why I was using it all day long, and the Wi-Fi emissor was pretty far from my position. Now things have changed to normal conditions, and I'd say the battery life is pretty much the same than on iOS 6.
Probably, I will end selling my iPhone 4 in order to upgrade to an iPhone 5C, but not due to a bad iOS 7 performance (It's not bad at all), but for the slowness when it comes to open apps, shoot camera, and the 4 inch screen look very atractive.
As for the animations: No, it doesn't have the weather app animations indeed (but who the **** changes their phone because of this?

), BUT it does have translucent dock, the notification center is somehow transparent (different from other devices), and the Control Center is grey, but I don't see a great problem on that.
If you (all you, dear macrumors comunity) need any other detail of iOS 7 on iPhone 4, I'll be pleased to help you.
Regards.