Disc brake rotor break-in
Disc brake rotors straight from the factory do not have an ideally smooth surface. As opposed to rim brakes rough rotors heavily decrease brake performance.
After only a few braking actions the grinded surface is still well visible and braking force is insufficent:
Rotors and Pads need a brake-in. Depending on Rotor/Pad material and roughness of the rotor surface 100 or more braking actions may be necessary to remove all of the scratches.
1 braking action = let the brake drag until the rotor gets more than 100°C hot and cool down again with water (evaporating water has a cooling effect).
Repeat until the rotor surface is completely free of any scratches when inspected in bright light: