This seems completely daft because one of the "selling points" of the D78s from day one was quick door operation wasn't it?
It was, and it's an utter failure, the doors are far slower than any existing UK DMU, even the Class 153 which is probably the slowest ex-BR DMU.
Even the panels are awkwardly positioned, facing into the coach rather than sideways, so the guard has to go out, do their check, fully reboard to find the close button, press it and go back out again, that seems minor but it costs a good couple of seconds per station, and on local stopping services seconds count.
If the door speed was the same as on LU and driver release had been used, and the button went green as soon as open was pressed[1], I reckon you'd easily get 5 minutes off an end to end MV run, as it is they lose time badly on existing 15x schedules, and this is the main reason for it.
[1] The slight delay probably loses about 5 seconds per stop, as the passenger presses it, nothing happens, then they wait a few seconds before trying again. Again, minor poor design costs valuable seconds. This seems to be adjustable in some way, as not all three units have the same length of pause.
Do they have the same nasty screechy door open/close tones as the rerfurbed Sprinters?
I can't remember to be honest.