Apart from the mini 3, those are all sporting the latest available Apple CPU.I have a mini 3, iPhone 6 plus and iPad Air 2. I'm not seeing any major issues with iOS 8 on any of my devices.
Mind, we have iPhone 5, 5c & 5s, and iPad 4 & Air in the household all running iOS 8.2 and we haven't noticed any significant slowdowns in comparison to iOS 7. Granted, we do have Reduce Motion enabled on our devices.