[bdickason]

My follow up question would be: Would a capacitive touch screen with 0.5s latency have made this the killer feature? Or did the capacitive touch screen enable high speed input?

I'd argue the latter, but it could be a question of framing?

[atleta]

I don't know. Again: I think the killer feature was the whole UX. Slow response times is definitely disturbing. (All my android phones got into this state sometimes.) You just don't get that feeling, but it couldn't possible happen to iphone because the the UX was the center of the whole product. I'm not an apple fan (never had an iphone, and the early ones kept pissing me off when friends asked for help) but it's obvious that they are obsessed about UX and polishing the UI.

But you are right that the capacitive display itself makes the interaction faster because it's enough to touch while the resistive has to be pressed. So it's probably slower and feels like you have to put in more effort.

[mercer]

I'd say based on the number of friends I have who use old or cheap Android phones with terrible latency, it really was the touch screen and UI/UX that played the main role.

Sure, for many, iPhones are still preferred because of the low latency, so it matters, but I suspect the iPhone would've done just as well if the latency was bad. The competition was about finger vs keys/pencil, not latency.