If this were true, then EVERY user (even those with the newest device) should be sitting up and taking notice.
If the jump from iOS8 to iOS9 has caused even an iPhone 5S to lag due to a tiny animated clock, then you have to wonder how badly optimised and coded it is. Sure, you may not notice it on the latest device, but all Apple are doing are throwing better hardware at poor software (which covers up the lag, just as long as the new hardware can make up for the poor software). We are into Android territory where powerhouse phones are required to make up for Android's awful optimisation.
Everyone would benefit from optimisation in terms of the user experience/battery life/etc - there is literally no excuse a tiny animated clock can cause so much lag. What's more poignant is that there really isn't much difference between iOS8 and iOS9 in most areas. Just simply the homescreen is identical - swiping shouldn't cause any lag at all, just like with iOS8.