Re: iOS 8 impression
I have iOS 5 on my iPad 1. It introduced some severe hesitation, which was never corrected.
... but at least you can try an alternative keyboard? It's unlikely to help but you never know.
I decided to sacrifice on of the iDevices (mainly to the God of curiosity :) ).
Initially it lagged, but that disappeared later. Not sure if that is because it had to re-index the device, but the lag is less now. I can't tell you much about the predictive keyboard - I reckon it has to learn yet and I generally use iDevices for consumption rather than creation so that may take a while.
For the rest no real issues with functionality yet other than that they added some "Tips" thing that is impossible to nuke, and worse, by default hooked up to notifications so it disturbs you with, well, really simple things. I went through its content (8 units, no real surprises), then stuffed in a folder I don't intent to look at (it also contains the News stand icon).