What effect does an airfield, at altitude, have on an aircraft's performance. For example Nairobi and Kathmandu?

On runways at a higher altitude an aircraft's engines are less efficient effectively giving the aircraft a reduced lifting capability resulting in a lower Maximum Take Off weight which means reduced payload and the requirement for a longer take off run. The same effect is caused by very high temperatures even at sea level. The three factors that determine an aircraft's performance are therefore Weight, Altitude and Temperature or WAT limits.