The certification requirement is 90 seconds WITH HALF of the exits blocked or inoperative. Looking at actual evacuations, the exit restrictions in the rule do seem to align very closely with the surprise factor in real world evacuations. Look at the widebody Airbus that went off the runway in Canada. No advance thought that anything was wrong, so maximum surprise factor. The miracle on the Hudson had more than a minute of gliding flight (i.e. something not normal) before hitting the water. Many of the passengers texted their loved ones on the way down.