The towers came down at as close to freefall speed as you can get - with absolutely NO resistance whatsoever. Without the use of explosives, it would have taken those towers at least 45 seconds to collapse. However, in reality they wouldn't have collapsed at all, as there would've been too much resistance from the 80 or so floors below the impact point. The 30 floors above the impact point would have come down, but not the whole building, and certainly not at freefall speed.