Your brain can actually slow relative time in tense situations. They did a test with a digital number board, in which the numbers were flicking by too fast too see under normal conditions, but for the first time bungie jumper it was easy to distingish the sequence. What is actually happening it you are seeing faster then normal, kind of like a video card, normally you get 32frames per second, but overclocked you get 60fps. This is also why people in tense situations report it feeling like it took place longer then it actually did. Time is just a chemical reaction within the brain anyway.