If you are using iOS 8 on the iPhone 4, make sure you turn on "reduce motion" and turn on "reduce transparency" in settings > accessibility > increase contrast.

If you are using iOS 8 on the iPhone 4, make sure you turn on "reduce motion" and turn on "reduce transparency" in settings > accessibility > increase contrast. That alone made my iPad 2 go from garbage unusable, to, ok. Transparency and blur effects seem to be the culprit for most slowness on older devices.