Well, if look the half full glass: iOS8 was slow and buggy until 8.4.1. even 8.4.1 is slower on the same devices than 7. iOS9.1 is just a little bit slower than 8.4.1 on most devices, but 9 has been pretty stable since 9.0.1.
So there have been improvements and hopefully this won't be an exception.
People seem to forget one thing: EVERY SINGLE iOS release since 2.0 was slower than the one that came before. Infact the slowdowns are getting more minor as time goes on. iOS4 was good on the iPhone 4, pretty good on the 3gs, which was at the time in the place where the 6 is now, and garbage, pure unusable garbage, on the 3g, which was in the place where the 5s is now. So, things have improved.
people tend to forget fast and see the past with rose colored glasses. That said, I'm not finding excuses for Apple, they said iOS9 was gonna be 1.6x smoother thanks to metal, so they dig their own grave. BUT...apart from that:
- 9 has been pretty ok since 9.0.1, where 8 always had pretty serious bugs until 8.4.1 which still has the discoveryd issue.
- 9.2, AT the moment is much smoother, than 9.0.x and 9.1