Which iOS relative to the initial iOS of an iPhone gives the best/fastest performance?

For example, if an iPhone is released with iOS x as its initial iOS, and which out of iOS x, x+1, …, x+5 or x+6 (the last iOS that supports it) typically give the phone the best/fastest performance?

Is it true that the further the iOS from the initial iOS, the worse/slower the performance of the phone?

Thank you for answers.