Surprised to see the negative feedback. I think it's a good point to at least bring it up. Apple already does everything they can to cripple new iOS releases feature-wise on older devices. For example, some of the killer new features of iOS 9, such as multi-tasking, were limited to the iPad Air 2 at first. With these kinds of omissions from the OS, in theory, the OS should perform just as smoothly, at the very least, on the older devices if they're supported. There's nothing new on the graphical/effects side, other than their backend switch to Metal, etc., so why iOS 9 would perform any worse than iOS 7 is beyond me.
I agree, it would be unfortunate if this leads to Apple offering less iOS updates for devices, but I'd love to see them put a bigger focus on making performance and optimization a bigger priority when releasing new iOS versions. Lately, it seems that they release these optimizations as minor updates several months after launch. For instance, I started noticing smoother animations on my iPhone 6 Plus on the latest 9.2 iOS release which was just released earlier this month. Why were the animations any worse than iOS 8 to begin with?
Just like with video games, frame rate and load times are some of the last items developers look at, but I think it really needs to be one of the first. It's a big part of the overall experience, and even if you have a perfectly polished product, if there's slowdown or you have to wait an excessive amount of time for something to load, it leaves a sour taste in your mouth.