Fire is unbelievably hot. Within a few minutes, a fire can generate temperatures of between 300 and 1200 degrees near the ceiling. One breath of this air can be fatal. However, even in these conditions, the temperature near the floor can still be under 100 degrees. This is why you must practice staying low and crawling out.
Fire is fast moving.
It can take less than five minutes for a small fire to spread and engulf an entire home in flames. There is time only for immediate escape. Awakening to the piercing alarm of a smoke detector is disorienting and confusing. The narcotic effect of the smoke can further numb your ability to think. That's why preparing yourself t through fire drills is your best chance of survival.