Plenty or reasons. 64Bit v 32Bit and larger, higher resolution displays with the same amount of RAM equals serious performance hits across the board.
Also, forget the new software features for a moment and accept that iOS8 performs worse than iOS6 and even iOS7. An iPhone 5 running iOS6 runs rings around an iPhone 6 and especially a 6+. Super smooth, no stuttering, no freezing, no delays, no scrolling issues, no website reloading issues, no app reloading issues. Just a plain superior device.
Funnily enough, it seems that I was wrong to expect better performance from a two generations newer model. Not only is the performance not better, it's markedly worse and apparently my experience isn't the norm.