Somebody needs to implement a Questions Requiring Thought (QRT) system. There should be a way to note questions during the interview that would require a lot of thought before a smart answer could be given concisely. Interviewees would leave the interview, put their thoughts together, and make a concise video of their answer. YC could review these answers anytime. I bet for 1.2x the time they're spending now, YC could get 2x the information.
Of course, this would depend on the availability of YC alumni. I know I'd sign up to do it -- if I were a YC alumnus, that is -- but the fact that I spend several hours per week acting as an alumni representative on my alma mater's university senate (and committees thereof) marks me as being unusually prone to volunteer for such things.
Yes, that is a problem for us too. We'd like to be able to spend longer with each group. But we want to give the maximum number of groups a chance to interview. If we spent more time on each interview, we'd have to interview fewer groups.