Just installed iOS 8.3 on my aging iPad 3 retina. It is not so bad. Actually seems kinda smooth. Everything rolls juuuust a little bit smoother. I didn't have any use in iPad for jail breaking anyway.
Here is a russian guy showing 8.3 on his iPad 3. Doesn't look too bad but hard to judge the performance only based on this video... https://www.youtube.com/watch?v=lwb9bc31jm0
This guy writes Safari and the keyboard still are much slower compared to iOS 7. And he didn't even updated from 7.1.2 but from 7.0.1 ... https://discussions.apple.com/thread/6997350
This is exactly why I don't understand the Apple update policy ... I either have to take the risk and test it myself or I have to rely on third party experience somewhere from the net I have to search for but then still can't be sure how it would de facto run on my machine ... I really would like to test 8.3 on my iPad 3, but not if that means I can't go back to 7.1.2 if I don't like the experience... So I think I'll play safe and stay with 7.1.2.