To slow a hard court down, you add more sand mixed in with the paint. This will make the surface more abrasive and slower. Grass court you use a different type of grass. Clay courts slow down depending on how much top layer there is and how much you water the court. For example, right after rain the court will be very slow, balls will pick up more of the clay, get heavier. If it's hot and a clay court isn't watered enough the surface will harden and get quicker.