I read the long post and I've not yet tried restoring my iPad2, however I noticed an improvement ONLY by changing the image of the dektop ...
Yeah!!
I used a photo (taken from the library) that for its size (about 8Mb) probably took up too much memory.
Replaced with a default desk image, the most annoying lag (jerky transition from one app to another in the task) are greatly improved.
it is clear, at this point, that each device has its own history that iOs 8 has contributed to exasperate.
I'm not sure that a full reset will contribute better because my old iPhone 4s whose default reseated without backup restore, but it has same lags and jerky as before.