Is to worth to upgrade?
Don't do it. If it weren't for an app I require for work, I would still be happily using iOS 8.x on my 6 Plus.
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If you're already on iOS 11, upgrade to 12.0 as you'll see some improvement, if you're on iOS 10, keep it that way until your favorite apps stop supporting iOS 10 (iOS 10 is definitely faster than iOS 12)
I'm still on 9. Haven't seen anyone compare 9 to 12, so I'll stay with 9, assuming it's faster.