I tend to think the same. I have a 6+ and it feels like Metal just hasn't been "turned on" yet? Really hoping this is the case. Also, after iOS 9 is officially released, I imagine many developers will make better use of the Metal API to speed things up even more over time.
I am extremely hopeful that this is the case. I don't know how you can just "enable" something like this though, if that were the case, Metal would have been adopted MUCH faster and on a much larger scale with developers. Doesn't it require rewriting and such? Maybe Apple hasn't put in Metal yet because they tried it "behind the scenes" during testing and it caused a lot of issues like tons of graphical artifacts and such due to graphics being handled differently, so for now they're ironing out those things before they fully implement it when iOS 9 goes public. One can only hope I guess, I highly doubt all of that.
All I know is that control center on my 6 dips into 40-50 fps where it is perfectly fine at 60 fps on an iPhone 5 running iOS 8. The frame drops really don't occur until the end of the animation, where the "bounce" effect occurs. Little jitters occur at other totally random places, like opening and closing apps, entering and exiting the app switcher, scrolling through the app switcher, etc. If this isn't fixed by GM I'm going to be very disappointed. Also would love to see the app switcher be fully smooth on all devices, like it was iOS 8 and earlier. My iPad mini 2 drops into 20 fps when entering and exiting the switcher or closing apps in the switcher. Scrolling is *okay* but it does stutter some, especially near the "home screen card." Control center and Notification Center stutter when used over the app switcher as well, happens on iPhone 6, too.