To get to 25,600 feet it was necessary for the pilot to be breathing at ground level rates. A rapid depressurisation and loss of oxygen at that altitude would have suddenly reduced the supply of oxygen to a quarter of that. The impact would be immediate. In one minute would come lightheadedness, dizziness. In the second minute, breathing becomes heavier and more rapid as lungs seek to compensate.
Peripheral vision becomes impaired.
You're starting to lose the ability to make critical judgments, but you are not aware that it's happening. After three minutes, disorientation becomes extreme. You are dying, but you are totally unaware of what's happening. After four minutes you become unconscious. After that, death is quick and painless.