I've actually had the battery give up the ghost to the point where when I was in class attempting a draw I didn't have a visible dot when I presented the gun. I was on the timer shooting for score, so some level of stress. Recognizing this and transitioning to the irons took maybe an additional .1-.2 of a second.
There are two possible conditions for the dot when I present the gun:
1. I see it
2. I don't
A big bloomed dot that is really, really bright I can still use with ease. It obscures more of the target downrange. If I'm shooting a B8 at 25, for example, I'm probably not going to be able to see the B8 if the dot is aimed in. That's fine. I can make sure I don't see B8 and know I'm breaking shots in the center. Or if it's dude at 25 yards and I have a big bloomy starburst, I just put the hottest part of the starburst on dude's chest and work the trigger.
If I don't have a dot because my battery is dying or because I have the wrong setting on the optic for whatever lighting conditions I'm in, I have the sights sitting there in the lower 1/3 ready to go.
It doesn't really matter to me. I'm looking for the first good go signal I can get. I don't care if it's from the dot or from the irons. Sometimes I have a visible dot but because of the way I presented the gun the irons are what I see first, so I go with that. Most of the time the dot shows up first because it sits higher than the irons. At speed, whatever tells me to "go" first gets the nod.