Well firstly platform staff don't show a flag to a conductor. And I am not making up anything
Which are you comparing ?
Dispatch given directly to the driver by a member of platform staff or dispatch given to the guard by platform staff. On my Guard dispatch services its insignificant between the two.
From when the first close doors is given both Driver and Guard hit the door close button - Zero time difference.
From interlock - The station staff check the platform for the train safety check and give the next indication to Driver or Guard. The time difference between me confirming two on the bell is barely a few seconds. There would be a difference between stock and dispatch procedures as I have noted on some TOC's the Guard needs to close his local door before interlock is gained.
On my DOO services it takes less than 10 seconds from CD to taking power. With a Guard its just as quick.
If you took away the platform staff then I would say that Guards can be quicker at times as I need to go through more safety checks and I need to make absolutely certain I am safe to proceed. When I have a Guard I am confident that when I get tow on the bell I am safe to pull away.