Would I, with my naked senses (eyes, touch) be able to tell the difference between a 55ms response time and 117ms response time? Alternatively, does such a difference add up or combine with other factors that ultimately make it noticeable to me? How? And while we're still at it, is this winning performance by Apple touchscreens a function of the quality of the screen itself (or components), other hardware (ICs and what nots) that work with the screen and are a factor in how it responds, superior code at OS level? I'd love to hear such details, as opposed to merely telling me 55ms vs 120ms.
In general, your visual response time is about 200 ms. Anything below that is fine for regular visual feedback such as highlighting a button after it has been pressed. This is due to latency in the processing of visual information in the brain.
Touch screens are somewhat unique however in that they track physical movement directly. This means that they do not only have to compete with the latencies in the visual system, but also with the predictions of physical movement that we do. Even though we can percieve visual information only at a 200 ms delay, our predictions of the physical world are compensated for that. Thus, even small latencies are percievable.
I think Microsoft did some research on this and they concluded that even single-millisecond delays in touch screen tracking were noticeable. They even claimed that sub-millisecond delays felt "completely different" and much more real.
Perceived audio latencies are much lower - I believe this is why designers don't include much audio cues in mobile UI's; you can do that with 3-6ms latency of keyboards/mice, but not with 50-100ms latencies.
