A couple of points there.
On the A350, the D1 slides are single lane slides with width restrictors to prevent excessive flow, whereas the rest are all dual lane*. At some point you have to trust the equipment you're given. One reason it took 8-10 minutes to evacuate (when they finally started) might have been due to the percentage of people sent forward. Had they used L4 slide to better advantage, and possibly R4 as well, the total evacuation time would have been alot nearer 90 seconds.
* There are packaging reasons for this, the D1 area is narrower due to the taper of the nose and it saves a couple or four inches of space each side. Besides, the front of the forward cabin may only have 12-18 seats (2 rows or less in economy terms). For example Our -1000 is configured with 44 suites between D1 and D2; 12 suites, 56 Prem seats and 49 economy seats between D2 and D3; and 176 seats between D3 and D4.
A high density -900 is still going to be arse heavy in terms of pax per door.
I would suggest that the final person leaving D4 did so with a degree of panic. You need to go with enough aplomb to keep momentum out of the door, but it's not some Hollywood movie looking for style points. Also CRM and pax management comes into play here. Delegate 2-4 people to support the slide as people come down it. Also required if the slide fails to inflate... Pull it taut and it will still serve its primary purpose. Delegation of certain actions to Able Bodied Pax is part of the preplanning process in an expected/planned emergency, ideally someone next to your crew seat. If they say no, swap them for someone who is prepared to do what's asked of them. Not so easy in an unexpected/unplanned situation, but still a useful tool in an emergency.
Similar documentation for the A380 suggests that the respective door sill heights on the main deck are comparable to the A350.which suggests a legislative requirement. Don't forget also that the slides double up as liferafts and so are a certain size to accommodate more than their share of the pax capacity (the opposite of the Titanic).