I remember the MK1 EMUs on the GEML. I think that you will find the open seconds had 2+3 seating, which meant there was only room for one person standing between the seat ends holding up the high luggage racks. A fully loaded train had poor dwell times as those who wanted to alight often were the ones who couldn't get a seat and had to step over 8 or 12 interlaced feet to get to the doors. Then the reverse happened, - those boarding had to step over the same feet. There are a few here who regularly wax lyrical over the MK1 suburban EMUs (ER & SR), but compared to sliding door stock, they were pretty poor in busy service, especially as there were so many regulars who insisted in sitting right by the door in 'their' seat, even though they boarded at the origin of the run and alighted at the end.
I've not seen any evidence for poor dwell times for high density stock. Certainly nothing to suggest that trad high density stock was any worse than people trying to battle their way from the end of a crowded 150 carriage to the door for example (something I experience regularly).