It could be the cores, or could be the advent of more than 1Gb of ram.
I am a long time android user (since when devices were equipped with 512mb ram) and android Os has kind of found the formula to smooth experience even in low ram, though now more ram (>2gb) guarantee smoothest experience.
I have just bought a 6s plus 128Gb (TSMC version). iOS 9.0.2 out of the box, the lag issue was present even in the home screen animation when scrolling through the list of apps, lol. This was like i am using a 1Gb ram Android 4.4. The lags and stutters were even worse in non-Apple apps like Path, it scrolls like a 512mb ram Android 4.2.
Then i have just upgraded the 6s plus 128 to iOS 10.2. The system lags get fixed but not 100%, still annoying though thinking that I am holding a $1k, branded, s-labeled device that scored 130k on antutu. The lags and stutters are still present in non-Apple apps, lesser in Apple apps.
So, First I am really disappointed with Apple just because of the lags and stutters that once ever happened on android os while in the same years never in iOs.
Second, i wish the devs found the "project butter" for iphone.