We bought a couple refurbished iPhone 4s, one for me and one for my daughter. She was OK being a guinea pig for iOS 8.1.2 (the little red notification was driving her nuts), so I updated her phone only. Side by side, two identical phones, one running 7.1.1 and another 8.1.2, my phone is noticeably faster in opening pretty much any app. That's not to say that 8.1.2 is unbearably slow, but it does seem to add a second or two to opening apps.
Until (if ever) Apple fixes that, I am staying with 7.1.1.