My slowest guard release from a correct stop on a SWT class 455 has been 15 seconds at Fulwell, some guards are really quick. My Pendo last month at Euston from Liverpool I'm sure the driver released the doors. Southern do the driver open and conductor close, which tends to be safer in my opinion because the driver knows they are safely stopped in the right place and don't have to worry about the guard not being on the ball.
I'm sorry but that's not entirely correct. Southern conductors
must release the doors on 171s working at any point on the entire Uckfield line (and indeed on services stopping at smaller stations on the Marshlink line where the train is more than 2 coaches in length). In addition, conductors must be competent to release the doors on any route they sign which can be worked by 455s, and must know the relevant platform lengths etc. This is mainly because the driver cannot release the doors using door deselect (171s over 2 coaches) / manual door locking (455s) where short platforms exist.
377s can't have the doors released by the conductor due to various software issues, but door release buttons are provided in the cabs next to the conductor's signal bell in case this is ever needed. Obviously that would mean that conductor panels would become near-obsolete as they only have door close & signal buttons (no idea why, it's just the way it is).
At Southern the rule is that the local door must be opened first, the conductor must step out and ensure the train is on the platform in the correct place, and release the right number of doors only when they know this. It is not strictly speaking mandatory to see the whole train in the platform at once, as there are some stations where this is pretty much completely impractical (London Road Guildford, Crystal Palace, Ashurst, Edenbridge Town etc.). However, it is mandatory to step out. This makes sure the doors are opened on the correct side as the conductor has to actually place their feet onto the platform, and also ensures that a short platform can be seen as a reminder before door deselect (etc.). Some conductors have been known to ignore such rules which is unacceptable and can fortunately end up being flagged up very easily. The train recorder systems will provide a replay of door opening, closing, sides released, door errors and so on, so any problem can be analysed minutely.