Before reset all setting my iphone 6 is same with 9.2, but after reset my 6 is faster like ios 8 i think.
Still does make it an absolute "must" that everyone resets all settings every time they installed an update like your original post suggested. It can work in some cases, but it's not a must for everyone, every single time. It's only if something is acting up and other "fixes" haven't worked kinda thing that anyone should use.