I'm speaking for the iPhone 6 as that's the only model I can test on a daily basis. I'm not talking about the iPod Air 2 and the 6s as I do not have those devices.
However, every iPhone 6 and 5s I've been able to use, has the same legginess to them. It's not the phone, it's that the OS isn't optimized properly for those devices.
I know it's not just my iPhone either because I've done DFU restores, restored from backup, setup as new phone, reset all settings, updated over the air, updated through iTunes, and even did a full restore as new with absolutely NO extra content on it besides the default Apple stuff and it was the same every time. Metal just isn't designed to run on this hardware optimally.
Well, all I can say is that metal-based apps like Vainglory, AG Drive and Geometry Wars, ran very well on my iPhone 6 in ios 8 and still do now. The OS hopefully just needs more optimisation to run better.