I was scared to upgrade, but I bit the bullet. My 6 is running perfectly smooth with no frame rate drop (I upgraded only an hour ago too). In fact everything seems faster as well. Maybe I just got lucky
I know it sucks but disabling Siri spotlight suggestions will get rid of this also helps some what with the app switcher lag, not really a good fix but for now it's works, and for me Siri suggestions aren't even working just shows my last 4 used apps=pointless
I have the biggest frame drops when closing Apps - animation showing home screen again is terrible in most cases - sometimes it freezes for about 1 second (iPhone 5).
For me, it has to be when opening an app, switching to another one, using the keyboard, touch ID, even when sending out messages there's an animation lag. And I have a 6!
I have ran into lag while listening to podcast, the whole iOS lags while I have podcast running in the back ground. Most annoying is a lag in safari typing in a search field.