We don't update software that promises "performance enhancements and optimizations for older devices" then get 98% of the old system. The minimum should be 100% what the system already offered, then go from there.
In old non agile days, companies shipped when system reached "maturity", not sooner as treating their customers as beta testers. This is the BS that started with "free services" and "ship it faster" mentality in Silicon Valley and sadly Apple seems to have fell victim for it too.
Customers don't buy products based on future possible optimizations, but how it performs NOW. If it doesn't perform NOW how it was yesterday following an update, they will complain as the product wasn't FREE and it's their right to do so.
You quite finished?
Where did I say people shouldn't complain, or did that minor detail slip your mind when your fingers began rattling the keyboard?
Check my post history, I've been one of the more vocal people regarding iOS 9's UI lag, but don't let facts get in the way of a good temper tantrum eh?
9.1 is miles better than 9.0, so right now I'm more content on iOS 9, especially on the Air 2 with Split View.
And if you expect a new version of any software to run identically to an old version that was 12 months old when it was replaced, you're being incredibly naive. 9.0 was never going to run like 8.4.1, in any world, any universe, or any other version of events you can fathom up.
Oh and, "One more thing" ... shouting individual words in capitals doesn't make them any more correct, relevant, or important.