Any cord adds impedance to a circuit.  That impedance is a resistance that causes the cord to release energy in the form of heat.  The longer the cord, the more heat is released.  That is why the building code required electrical receptacles to be placed a certain maximum distances allowing for proper coverage.  In most rooms, the code requires receptacles spaces at maximum of 12 feet.  That is why most cords are 6 feet long.  Kitchens require receptacles at 6 feet spacing so that is why your toaster or coffee maker has a 3 foot cord.  If you need more receptacles or one closer to your devices, it is better to have an electrician add a receptacle.  Extension cords are only to be for temporary use and are not to be run between rooms, through windows, doors or walls.