Why it take more time to …

On mountains and hilly regions, the air is less dense and the atmospheric pressure is also less. As the altitude is higher, the air column weight is reduced. So the pressure is reduced as compared to the sea level or below the mountains.  Water has a lower boiling point at a lower temperature.

The food items like rice or meat need to be cooked at high temperatures. If we heat rice at 70°C, it does not cook completely. It becomes half cooked. Usually we use water to cook our food. So water heats up to the lower boiling point (say 80°C) at the top of the hill and boils.  The temperature does not rise above 80°C.  But at the sea level, the water boils up to 100°C and so the food is heated to 100°C.  Then it is cooked nicely and it becomes soft and edible.

It is possible to add salt to water to increase the boiling point on top of hills. But salt water could change the taste of food.