iOS only seems stagnant because they don't have to trumpet every year about how they've finally gotten rid of ui lag..
You might not notice it, but, there is an eighth-of-a-second-ish delay between when you tap on most buttons and when the action occurs. An obvious (to me) example is when you tap on the back button on a navigation controller, and it takes a split second before anything happens..
It isn't much, but it is consistent across the OS, and it drives me crazy.