What held the Natives back?

[Sven]

In other words, "progress" (defined however you like) took a great deal of time?

No, that is not what I said. What I said was that time is not the only factor, a point you seem to agree with. So, I'm not sure why you're getting snarky about it.

As for early Sub-Saharan people developing slowly, well; part of my point was that "Old World" cultures had long experience interacting with other cultures (sharing ideas, tech, etc). Can't interact with people who aren't there yet and for which interaction the tech doesn't exist yet anyways.

Let's examine the claim for a minute. Your exact words were:

...which suggests that you believe the Old World's relative advancement was due to their "head start" over the New World. It certainly makes sense from a purely rational perspective that the Old World had an advantage because it was developing over a longer period of time. But, empirically, the claim meets a few problems.

The first is that "200,000 years" only applies to Africa: we were only in the rest of the Old World for about 100,000 years. This leaves the question of why Africa was "behind" Eurasia unanswered.

The second is that many of the earliest societal advancements don't show a dichotomy between the hemispheres. For example, current evidence suggests that agriculture started at pretty much the same time in Africa, Eurasia and the Americas (~10,000-11,000 years ago). So, Eurasia did not actually have a "head start" on agricultural development.

But, thereafter, crops spread rapidly in the Old World, but slowly in the New World. The Old World also ended up with more different types of crops than the New World. So, it would be more accurate to characterize the Old World's agricultural development as "faster," rather than "earlier."

Because of this, I question your claim that "New Worlders would have gotten there had they not been interrupted; they were already well on their way." They certainly were well on their way, but they were taking a much longer time to "get there" than Eurasia did. So, how much more time would they have needed? I suspect it would have been rather a lot.

So, why was technological advancement faster in the Old World? You mentioned interactions with other cultures. And, I agree that interacting with other cultures is a big advantage. But, it begs the question: why were there more cultural interactions in Eurasia than in Africa and the Americas? Was it because there were simply more cultures in Eurasia to interact? How did that come about?