Performance-wise it's ok, but then iOS is an operating system with such a low feature set that it would fly on older spec Android devices. RAM-wise it's old spec and really, apart from the bigger screens, there isn't much to distinguish the iPhone 6 from the late 2012 iPhone 5.
Performance wise it's near the top of the food chain. I disagree with your analogy vis-a-via the 5 vs 6. The post below yours is spot on. The feature set of IOS is right on par with the completition. It's just a different interface.