iOS 12 runs fine on my iPad Air 2, which has similar performance to the 6s if not worse in some areas (A8X instead of A9, same amount of RAM). But I've been hesitant to update my iPhone 6s Plus from 10.3.3 and lose any discontinued 32-bit apps. If that weren't a concern I'd have updated it to 12 already.
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Apple continues to sign the last supported version for use on that device, eg. iPhone 6 will remain able to update to 12, even though that device likely won't be supported by 13. But assuming iOS 13 does support the iPhone 6s, you have until at least mid September to decide whether or not to update to 12.