iOS 11 slowed down my 9.7" iPad Pro to the point where I practically leaped on the iOS 12 public beta just for the performance improvements. It was so bad that at one point, simply launching notability would cause me to lose connection with my Apple TV.From my personal POV I'm still running 11 on my 10.5" iPP and I'm not missing anything at all (other than the new hideous gestures and control centre). If 13 is another snoozefest with unnecessary iPhone X parity tricks, I'm sticking to 11.
I see no reason why you wouldn't upgrade to iOS 12. Even if your iPad technically has the horsepower to spare to bulldoze its way through any inefficiencies in iOS 11, why say no to free performance improvements?