Well...it's kinda hard to say. First a consideration: 8.1.2 is slower than 8.4.1. Which means iOS 9 won't be as slow for you as it will be for somebody coming from 8.4.1.
That said...it depends. I'm kinda sensitive to stutters but I paid a fair amount of money for my Air 2 and all its power is a bit wasted on iOS8. Want to watch a video in a small window while browsing the web? You can't. Want to do word processing? Very limited (no split screen, no trackpad mode, no keyboard shortcuts indicators). Want to sketch something on the note app? You can't. Want to airdrop something to your Mac? Too bad, airdrop sucks in iOS8. Want a stable network? Nope, you have discoveryd with you, your network connection at times will just suck. Want to upload a PDF from safari? You can't. Want to have bigger, more usable folders? Not in iOS8. Apple Music? Not in 8.1.
So...if you don't care about these features, stay on iOS8. To me, the Air 2 was made to be on iOS9. It sucks that it stutters sometimes...it really shouldn't. This thing is almost as fast as my 2014 MB Air.
Anyway, FOR ME, the tradeoff is acceptable. Almost mandatory. iOS is still too limited for me, iOS9 is barely enough to be useful. I still miss many features...
What you could do is wait for 9.2. I'm on the second beta right now and the smoothness is better than on 9.0/9.1. I think 9.2 will make many people happy if it keeps improving.
Finally there's a third way: you don't have to choose between features and smoothness. You can have both if you choose to sacrifice animations (reduce motion option in settings). Try it in iOS8. If you like it, you can upgrade And be almost without stutters, except in spotlight