Major purpose of iOS 9 is maturity.
The irony of this post is that I had to hit the safari icon twice to get here to respond to you. I'm on iOS 9.
Also, iOS 9 being a "speed and stability" release is a PR move to get people off their back about how inconsistent their software has been.
Unfortunately, when iOS 9.0 comes out and it's a lagfest, the intelligent people will realize they were lied to, while the rest finish their glass of cold Apple Kool Aid and claim the release as "buttery smooth".