I think this summed it up best. You want the math to back it up, you say: Change in velocity is governed by dV=at (acceleration times time). In this case acceleration is provided by (the local) gravity (g), so dV=gt. Slowing down sooner means your descent takes longer, so gravity has longer to act, as already stated. So if t (descent time) is, for instance, doubled, then the dV accrued from gravity also doubles, meaning your ship needs to expend twice the dV to counteract gravitational acceleration. I was also unsure why a suicide burn would be better until I read wasmic's post, at which point I had to smack my forehead and think "Of course, duh! that makes total sense"