I agree totally with Willc's point at the larger and busier stations, but HST▸ 's are a problem at unstaffed, quieter stations - Kingham, Malvern and so on - as even if the TM‡'s pretty sure nobody has got off they still need to make sure that all doors are shut which can involve quite a walk (anything on the first catch facing away from you is very difficult to see unless you're up close) and thus takes up precious time. Compare the dwell times on a trip from Oxford to Malvern with those of an Adelante with it's similar door locations and you can see why so many minutes have been added to the schedules since those days.
Precisely, the main reason 180 dwells were longer than necessary was because the doors were faulty. With a fault free 180, a despatch from WOF could be done in 3 minutes. With the HSTs, it's 4 if you're lucky, more likely 5. Why? Because on a 180, you needed a despatcher to stand in one place on the platform with the guard in the penultimate coach, they can see all doors and the guard closes them; on an HST, the guard and depatcher have to walk up and down opening (for the people unable to read) and closing (for the people who haven't twigged the doors won't close by themselves) doors. The guard is also positioned badly at the back of the train due to the
SDO▸ = poorer view = more walking. With the curves, a door on the catch is difficult to see. You also have the stupid situation where a pair of doors at the busiest part of the platform are platformed, but are not opened due to
FGW▸ getting the SDO wrong. The result is people try both doors - neither work = delay as they move down a carriage + delay as guard/ despatcher runs over to the doors, kicks them shut and walks back.
The whole procedure is a horrendous
mess and an embarrassment to observe. Even at Shrub Hill, which is dead straight, long enough and not that busy, a dwell of less than 3 minutes is rare. Plug doors could see 5 - 10 mins taken out of the timetable straight away.