It always amazes me how subjective adjectives like "slow" and "unstable" are when referring to software. I too have a 6s (plus), and iOS 11 has been just as fast and stable as iOS 10 in my experience.
This kind of disparity makes a lot of sense in the PC world, where everyone has slightly different hardware. I can't quite figure out why it's still common on a platform where there is relatively no hardware variation.