If iOS 9 is not going lightweight, I think apple and developers are going to undermine user experience gradually over time.
I am not a developer, even a decent programmer. I can only say, optimisation is necessary for any computer programs, as long as they are still in support cycle (even a week). IPhone hardware itself does need optimisation as well, although few members may mention it. To complete a "not" function, you can either use a super complex circuit, or simply several nand gates to achieve this.
Apple wwdc mentioned that "making a product bigger is not a good design, although everyone can do this. What we need is to let product become more powerful and smaller." I am sorry because I forget the exact words.
So, iOS is the same. Apple need to polish it, make it runs more fluent, and faster. Windows 8 is Microsoft's polish on their windows system, too.