The problem this is major issue is because:
- Apple promised performance improvement on metal devices (from 5s and iPad Air); clearly, the promise didn't deliver.
- IPhone 6(+) and Air 2 is only one year old and very capable. Even 6s is not free from lags (I don't have those devices, just stating the complain from threads here).
- It's only natural that OS get slower from each iteration. But iPhone 5 is smoother than 6+? That's something amazing. (Source: family devices).
Conclusion: some are deeply disappointed; because: Apple didn't deliver the promise.