Has been happening with x.0 releases for years.
Unfortunately, the x.0 release can be as long as half a year... in other words, half the time you own the phone you'll have bugs and poor performance.
Or, they could let you downgrade to the previous version that worked well.
iOS 9 has been pretty bad on my iPhone 6 so far. The thing that kills me though is how long the touchscreen and home button go unresponsive for during and after every animation.
There's no reason apps like Settings and Messages should be any slower now than the were on a 3GS back in the day.