Water boils at a lower temperature at a low atmospheric pressure so it takes longer for food to cook.
For water to boil the kinetic energy of the water molecules must be equal to or greater than the kinetic energy of the air molecules pushing down on the surface of the water due to atmospheric pressure.
If atmospheric pressure is low the kinetic energy and temperature of water needed to boil is also less. Because the water contains less energy it can not transfer energy as quickly to other objects. Because it can not transfer energy as quickly, because it has less energy, water must boil longer to do the same amount of work.
Water boils at a lower temperature at a low atmospheric pressure, so it takes longer for food to cook.
Water boils when its molecules have enough kinetic energy so its vapour pressure equals that of the atmosphere.
When the atmospheric pressure is 1 atm, water boils at 100 °C.
When the atmospheric pressure is less than 1 atm, the water molecules do not need as much kinetic energy for the vapour pressure to equal the atmospheric pressure.
The water does not have to be heated to as high a temperature, so the food takes longer to cook.
For example, a typical atmospheric pressure in Denver, Colorado, USA is about 0.83 atm, and water boils at about 95 °C.
It takes longer to boil an egg in Denver than at sea level.