I don't believe the reason for that is UI responsiveness (also, I don't agree with your 'primarily' assessment, but let's not chew on that). In my opinion, the differences are so small that UI performance just isn't an issue. There are other areas where iDevices are noticeably better compared to every competing device.
>What bothers people is that when you swipe from screen 1 to screen 2, there can be a > .5 second delay
Hm, I don't remember that ever happening to me in the last ~2 years with two devices that I've used most: nexus one and motorola defy. But ok, I grant you that it can happen. So what? I've also seen an iPhone freeze completely.