If anyone wants to see some real FPS drops on their 7 Plus (don't have a normal 7 to test) bring up a 3D Touch menu in the Music app and then, with the 3D Touch menu still on screen, open the App Switcher.
Scroll. Enjoy that.
The next biggest drop I've seen is if you have Siri open and then bring up control panel at the same time. Again, not micro stutters, very noticble FPS drop.
I wonder if it's anything to do with the new style of quad-core, with the two less powerful cores handling normal light use but not quite being up to task if there are a lot of the iOS blurring effects on screen. I only wonder this as the 6S doesn't suffer from it at all - unless you enable Low Power mode and the SoC performance is limited.
7 Plus is still killer though, being able to invoke the telephoto lenses while recording video is brilliant.