Listen, I see lag during your "I don't have any lag" videos. You're simply blind. I honestly wish I couldn't see it as well, but I guess I'm one of the unlucky ones.
The fact of the matter is, it didn't matter how many times I installed iOS 8, it never lagged like this. When a fresh restore lags, you know something is wrong with the OS, not my iPhone.
Does everyone have the same lag in the same places to the same degree all the time? Or are there differences as to the degree of the lag at least, if not the places and/or frequency?
Everyone with a fresh install will experience the same lag, but different software configurations will obviously have different results. There could be something corrupt in your backup file that iOS 9 doesn't like and makes the lag worse, and turning off transparency effects can help alleviate some of the lag (but not eliminate it).
My point is... it's not something the USER is doing, it's Apple's problem to fix. However, I believe everyone's iPhone 6 will have the same amount of lag (or more) as my iPhone 6 has. Same CPU and GPU combination and same amount of RAM. One person's iPhone 6 isn't going to have a different version of iOS 9.0.2 on it as mine.