My 6 is smooth. It's also my first iPhone, after using numerous Android powered flagship devices. Trust me when I say that I don't see any "lag" or "stuttering" with this "dud" of an SoC and 1GB of RAM. In real world usage, day to day, the difference between the 6 & 6s will be negligible. Get over the benches and white papers. I agree with your statement about the 6+, the "lag" is due to a higher resolution display that the internal components struggle to drive effectively. Like I said, my 6 is smooth; I don't know what people do to these devices to make them slow, but mine is brilliant. Eight hours screen on time today, with more than 20% remaining...something that would have never happened on a flagship android powered device.
Forums, man.