iOS 7 runs fine on my iPhone 4
Reset my iPhone w/o iTunes then updated OTA to iOS 7 and then reset it again.
I have to say, I'm impressed iOS 7 hasn't made the iPhone 4 unusable compared to updating the iPad 1 to iOS 5 which caused it to slow and crash significantly. I feared iOS 7 would totally make the iPhone 4 unusable. Its slightly slower than before, but no crashes and the animations while slower don't stutter.
Passcode
Entering a passcode is a little bit faster now, mainly to do with how the numbers are shaped now. iOS 6 tight square-spaced numbers made it harder to enter, its much easier now with iOS7's circled and more spaced out icons. Icons appearing takes a second longer due to the new animation.
Notification Center/Control Center
No lag what so ever. Yes, it doesn't have the blur effects newer devices have. Spotlight to me is better than iOS 6. There was a little bit of lag when you swiped to the left on iOS 6.
Core Apps
This is when I begin to notice a little slow down.
Phone, Mail, Messages, Photos, Weather, Clock, Notes, Reminders, and Game Center I noticed a 2-3 second delay. Game Center was by far the worst, took about 5 seconds to load.
Safari, Music, Camera, Maps, Videos, iTunes Store, App Store, and Settings I felt were either able to be a little faster or at least maintain its loading time considering all the new features added.
Battery Life (charged it over night, started using it at 7:30 AM)
100% - 7:30 AM
50% - 3:16 PM
iPhone Usage Data (Time Since Last Full Charge)
Usage - 3 hours 23 minutes
Standby - 7 hours 38 minutes
-Wi-Fi and 3G turned on
-Bluetooth turned off
-Brightness set at 50%
Apps
I only downloaded apps that supported iOS 7 and had newer UI elements. Most of them worked fine, Facebook had the worst lag and took a good 6-8 seconds to load. Evernote, OpenTable, Pocket, Shazam, and TED were fine.