I used to be against that apple was making older devices slower, but watching how poorly my iphone 6 plus runs with IOS9, I believe it to be true. There is no reason that an iphone 6 should choke. My 6S runs everything perfectly smooth.
That's because the 6s is wickedly fast.
The A9 is phenomenal.
If the lag issues are actually because of Metal on the A8 or A7 and it exists on the A9, it wouldn't matter because the A9 is just too fast.
Or maybe Apple tweaked iOS 9 in favor of the 6s because it's the new iPhone. Who knows.