The moment you turn your oven on, it starts getting hot, but most take a full 20 minutes to be fully preheated—even if the indicator light (or chime) says it’s ready sooner.
The indicator signals when the air in the oven is hot enough, but for it to be well heated, the walls also need to be hot. If they’re not, most of the heat in the oven escapes when you open the door. Put in a chicken in to roast in a cool oven and it starts steaming instead of roasting. In the end, you might find yourself with a dry chicken because it lost too much moisture during the first few minutes in the oven. Plus, without enough heat to activate the chemical reaction of cooking (the Maillard reaction), that chicken just won’t brown properly or develop the rich flavor that makes it taste so good. A “too cool” oven isn’t the only reason to wait—if you rush, it might actually be too hot. When an electric element is heating, it’s often super-hot. Put in a batch of cookies while it’s heating and the bottoms will burn.
Most recipes instruct you to turn the oven on before starting any prep, unless they have long marinating or chilling times. If you get into that habit, your oven will almost always be evenly heated by the time you’re ready to use it.
The best way to know if your oven’s at the right temp is to check it with an oven thermometer. If it’s not right after about 20 minutes, check the manual—calibrating it is usually easy to do without a call to the repair guy.