Something else I noticed on the 5S is if you have 20-50 in your recents (phone app), when tapping the upper part of the screen (to make it scroll automatically) it actually drops frames. It never did that on iOS 7 and the interface is exactly the same as iOS 8 (8.1).
Yeah, this is planned obsolescence at its finest or just terrible coding by Apple. For all I care it's the same anyway in this case. The A7 is plenty fast enough to handle iOS cause the truth of the matter is that the amount of functions and overhead in iOS is nothing compared to the ridiculous increase in processing power (Apple mentioned 50X since the first iPhone)?
There's no reason why iOS 8 shouldn't run as smooth as iOS 7 on the 5S and if they don't fix this soon, they I'd definitely say they intentionally slow their devices on purpose.
The iPhone 5 I have next to it running iOS 7 (7.1.2) obviously doesn't drop any frames when you make it automatically scroll up in recents (phone app). Buttery smooth.