Also happening to me, I have a iPhone 4S and this has been going on since 7.1.2, after some update. My Spotify iOS app now takes around ~30 seconds (!!) to load, and when it eventually does, it takes a long time before I can go through the menus or even scroll through playlists, let alone set it to play. It's clunky and constantly freezes. Closing and reopening the app sometimes helps but not much.
Which means that nowadays to get it loaded and playing, takes at least 2 minutes.
With iOS 8.0 got even worse, although with 8.1 has gotten slightly better.
I have tried deleting and reinstalling the app many times, change settings around, nothing works.