They don't make iPhones slow. They have some the fastest performing smart phones on the market with the A-series chips. Let's not blow this out of proportion just because of what you're reading from some users on a tech forum. There is a world outside MacRumors and those will never notice these types of tendencies at all.
But they do make phones slow. They slow down the clock speed. And then it's not one of the fastest performing phones on the market. Not even within its own generation of competitors.