What are you talking about, slows the phone down.? I have not personally seen any performance issue with iOS 7 on any device.
It's pretty evident of performance degradation on the iPhone 4 with iOS 7. Apple themselves are addressing it to an extent in iOS 7.1, if it was fine why fix it?
Plus Apple disabled graphic effects in iOS 7 for the iPhone 4 to aid in performance. Again, why do that if there wasn't an apparent problem with performance?
Comparing my 4 and 4S is literally night and day. Dual core saved the day .