What makes a hard court slow is a relatively rough and, much more rarely, somewhat soft surface. When the ball strikes a rough surface, it encounters more friction to slow it down and more and larger vertical protrusions to push it upward. The roughness of a hard court increases with the quantity and grain size of the sand that is mixed in with the paint topcoat. Most hard courts are too firm to be deformed even momentarily by the impact of a ball, but the softest, rubberized hard court surfaces can give enough to make them slower, because when the ball strikes a soft surface, it forms a little depression and is then slowed and pushed upward by the front slope of that depression as it exits.
A rough, slow hard court can be highly abrasive to balls and shoes. On some courts, brand new balls will lose most of their fuzz in less than an hour, and new tennis shoes with relatively soft soles will be more than half worn through after just a few matches.
Rubberized hard courts are much gentler on balls and shoes, but they can be tough on legs in hot weather if they become so sticky that they grab onto a shoe more than the player expects. Players who are used to sliding on clay sometimes try to slide on hard courts too, but while fast hard courts may allow some sliding, slow ones make it quite difficult and risky, as what starts as a slide can end quite abruptly. Legs also have to work harder by running down more balls on slower hard courts, because hitting winners to end points quickly is more difficult.
Longer points make arms work harder too, and the higher bounce on a slow hard court typically forces players to meet the ball higher, where the strain of executing a stroke is greater. The slow hard court's relative unfriendliness to low slice drives, which skid less and sit up more on the rough surface, further increases the average height of contact.
