There are, of course, inherent differences in the way individuals process information and in their base knowledge of any given topic. This has a lot of impact on the speed with which they are able to answer you in conversation.
But the real differentiator, as I have read and experienced in the past, is the amount of preparation one has done in the specific area being discussed. I think that this is a bit different from just the amount of knowledge a persons has and is more about how often they think about the topic on an ongoing basis.
For example, people who think about their work (software comes to mind) all the time, even when they are not actually working, will often know their area inside out and be able to answer questions very quickly. And those who are also lateral thinkers can often extrapolate an answer in a related area with reasonable results, further enhancing their "quick thinker" image.
That harkens back to Einstein's comment that genius is 1% inspiration and 99% perspiration. I.e. do the work.
2For me, watching "Whose Line Is It Anyway" (British version) helped a lot. Other improv programs (or even attending an improv class/group) probably would too. However, if you find yourself constantly re-reading text many times, you may have a form of obsessive-compulsive disorder, and should at least talk to a doctor. – barrycarter – 2011-10-10T05:10:47.583