in the hills the atmospheric pressure is lower than that in the plains and therefore water boils at a temperature lower than 100oC causing an increase in cooking time

due to low atmospheric pressure on the hills, the water boils at a temperature higher than 100oC and therefore water takes longer to boil

in the hills the atmospheric density is low and therefore a lot of heat is lost to the atmosphere

in the hills the humidity is high and therefore a lot of heat is absorbed by the atmosphere leaving very little heat for cooking