I'm convinced that different individuals have different sensitivities to these frame rate dips. I've taken my phone in and was told it's normal. TWICE. I can reproduce them on all display models.
I finally gave up. It's just a fact of life. I hand the phone to my wife and she has no idea what I'm going on about.
If all the display models do it, what were you expecting them to do for you? Rewrite the OS while you wait? Did you give feedback to Apple?