Two major positives I’ve noticed in terms of system animations. I updated a family member’s Air 1 to iOS 11 some time ago - this is the first version where pulling down notifications doesn’t exhibit any lag, nor does swiping to open Control Center. Very smooth now. Can’t speak to anything else though.
I have it on my iPad and it’s much better than previous versions of iOS 11. I don’t think you would miss much on iOS 11. Especially for iPad Air which has limited new multitasking functionality amongst all iPads.
updated to iOS 11.3 today wife iPad Air 1, butterly smooth performance, only hiccup was using iTunes 12.7.4 (possible bug) transferring multiples large photo directories was lagging.