Slowing an aircraft down - it is a matter of energy levels. High speed and high altitude aircraft possess an abundance of both Kinetic and Potential energy. As profile drag increases as the square of IAS, then increasing drag (Speedbrakes, etc) at high speed will destroy more energy than it will at lower speeds. Slowing the aircraft down and then using speed brakes will not have as much effect as using the speed brakes at high speed before reducing.