The sand content defines the speed of most grass and hard courts. More sand = slower court. Another factor is how polished the top layer is. If its gritty the court will play slow if its polished it will play fast. Newer courts are also much slower than older courts.
I play in Miami / Delray Beach every year and once they resurface the courts you automatically notice the difference from last years layer which has had much usage. The balls get very fuzy and its nearly impossible to hit through the court after 2-3 games with new balls.