Root vegetables will cook much faster if they are arranged in a shallow layer in a large sauce pan than a deep layer in a small one. Add just enough water to cover and use a pan with a tight fitting lid to hold in the steam.
On frying or grilling meat, poultry or fish, make all the slices the same size and thickness, as much as possible, so they cook at the same rate. Flesh can be cut in to strips for easy cooking.
On stir frying, get the pan hot as possible in a high heat, then add oil and only add the ingredients when the oil becomes hot enough so they cook very quickly. Be sure to move the food around constantly, so that it does not have time to steam.
Cooking food in vapour rather water or oil reduces the loss of vitamins, two or three steamer baskets can be piled up to cook various items at once.