What I got out of that video is ios 6 wasn't fast and ios 9.3.5 wasn't fast either with some functions being equally as slow. I would rather use 9.3.5 given that it has more app compatibility.
As far as my 6s, the performance is fine under 11.4.1.
@Radon87000 posted a video comparing ios 7 vs ios 11, claiming ios 7 is faster. It was faster in some instances, slower in others. If the instances it's faster, are my use cases, ios 11 is faster. If the instances ios 11 is slower are my use cases, I would think ios 11 is slower than ios 7.
Different perceptions, different people, different use cases.