As you ascend in elevation, atmospheric pressure decreases and water boils at a lower temperature. For every 18 °F drop in the boiling point of water, it doubles the time to cook food. For example, the boiling point of water drops from 212 °F at sea level to 194 °F at 10,000 feet. It takes twice as long to cook raw food at 10,000 feet than it does at sea level. So, if you are planning to cook raw food at higher elevations, be sure to bring extra fuel. For boiling water and rehydrating foods, altitude doesn't make much difference. It actually takes a little less time and fuel to boil water (since it boils at a lower temperature), but rehydration will take a little longer, which balances it out.
But none of this matters I would think if you want your morning coffee at over 13,000 feet.