This statement is constantly made in relation to end door stock but to what extent is it end doors that are the problem?
I would suggest that it has become an article of faith without direct comparisons ever being made. Mark 3 stock has a narrow (ie short distance from nose to riser) step with has to be negotiated. Both mark 3 and classes 153/156/158 have small vestibule areas with "pinch points" between them and the saloons.
I would also question where the data has been derived from. The original BR sliding door stock eg classes 313 and 455, had 1.2 metre wide doorways with no standback inside the doorway, thus passengers would adopt the "sentinel" position on either side, leaving room for only for boarding and alighting in single file. The Bombardier plug door stock suffers from an awkward entrance step. Later stock, eg classes 376/378 with sliding doors, has good standback space and obstructions to the free movement of people have been reduced, which unfortunately also causes more draughts when the doors are open at stations. Since the trains are used on short distance routes and passengers are wearing their outdoor clothing this is not regarded as objectionable.
For station dwell time, a fleet of slam door trains is unbeatable, with 15 second stops being achievable. But for the system to work passengers had to be more disciplined and there were of course safety issues.
Probably the most useful feature to reduce station dwell times would be a retractable step as shown
here.