Fair point, but would it actually be '7 minutes or more'? From my observations of 444s splitting and joining at Southampton the whole procedure seems to take no more than a minute, meaning the front portion could presumably get away in 2-3 minutes (start process 30 secs or so after arriving, 1 min for splitting, another min for embarking/disembarking) and then the rear three minutes later, i.e. do the whole thing in 5-6 minutes.
However it's a fair point that even 5-6 minutes would be too much. Presumably in the old days they could get away with it as less trains were using the station.