Can't think of a way that brains doing this is adaptive in an evolutionary sense.

Some hypothesize that flashbacks might be the brain searching for relevant useful memories, or hallucinating if it can’t find any. Or, perhaps emotions or physical issues cause your brain to function differently and it’s not an adaptive trait.

Time slowing down does seem useful in the event you can actually affect your circumstances.

Hard to select for adaptive traits for something that only happens just before you die.