Um blur is what makes the design beautiful in my opinion. Maybe people should stop complaining that high end features aren't working well on phones they bought in 2011&2012
I agree the blur is very aesthetically pleasing but it's not just old devices that it's causing performance issues with. The 6 plus and 6S plus are both experiencing performance issues here as well, most likely due to the increased demand from the high rendering resolution (1242x2208) when compared to its 4.7" or smaller counterparts. I think users with these phones have a reasonable complaint considering these are the new top of the line devices.
Fingers crossed Apple does some performance tweaking of that effect to make it more resource efficient. Smooth and responsive interface interaction has always been one of the primary elements that attracted me to iOS over Android.
The 4.7 inch iPhone 6 has minor issues with the blurring here and there and I've even seen small hiccups on the iPhone 5. My iPad mini 2 has NEVER done well with it even on iOS 7.x.x. It's just so annoying how Apple can't get it right, it's a taxing process but not *that* taxing is it?
The iOS 7/8/9 design would look awful without the blurring but I think it causes way too many issues for it to be worth it. Change the design to not need to blurring so much, or limit the unnecessary use of it. Like the keyboard, app switcher, pop up menus, spotlight, power down screen, headers, text entry boxes... It's too much. Control center, dock, folders, passcode screen, Notification Center, and that's about all you need.