My dad installed 11 on his Air 1 recently by accident and as far as I can tell it seems to perform similar to 10.
I agree - mine is working about the same as with 10.3.3 or whatever was the latest prior to iOS 11.0 Overall, the Air1 is starting to show its age though....for sure. Im gonna hold out a little longer before I jump on a 10.5" pro. I think.