"Decisions made in 48 hours" would be extremely helpful for startups. Since YC only skims written applications for a few minutes and interviews applicants for about 10 min, it seems possible for them to make quick decisions.
They probably do process each application quickly but they get ~10^3 of them. Even if they spent 10 minutes total on each, that'd still be ~6 days of continuous work. Factor that it is probably 1500 or 2000 and the fact that they can't possibly work 24/7 and you get about the time they take to respond.