I also installed iOS 8 on my first gen iPad mini, and it is now so slow it is unbelievable.
Previously I had iOS 6 on the iPad, and it run FLAWLESSLY but decided to trust Apple and update, and now my iPad mini performs like Internet on dial up - garbage.
Thanks Apple, you well and truly screwed me over. You have rendered a perfectly working device to one that is now too annoying to use.
And yes, everything has been tried to get the iPad functioning again, but no, Apple in their profiteering wisdom decide not to allow their consumers (you know, the ones who keep their shareholders happy) to roll back IOS and make a perfectly working machine capable again.
I'm off to android...