If you want the train's wheels to start moving pretty much bang on the departure time then 30-40 seconds on a HST▸ is about right, that could (and should) be reduced by 10 seconds for 180s (and other non DOO▸ trains including the new IET▸ trains), and could (and should) be reduced by another 10 seconds for anything running DOO.
Certainly adding another 30 seconds onto each dwell at Melksham this afternoon with the number of mobility restricted people getting on and off the train ... young children and pushchair day. Total of 56 passenger off and on the round trip that called at 15:39 / 16:37, but ticket number will show less because lots of under 5s.
Personally, I would like the
GBTT▸ time to be the time the doors start to close; why does the rail (and the airline) industry quote a time that's operational focussed rather than customer focussed?