Public courts that aren't resurfaced often enough just become fast by wear. Sunlight does plenty damage in Florida to expedite the speed of the courts. The surface paint uses nowadays is grittier and thicker than it was a couple decades ago. The result is that courts with fresher surfaces is slower.
However, clay courts that I play on in the humid southeast tend to have a wetting effect of the ball. So, I am not sure whether the clay is slower, but the heavier balls make clay tennis down here seem slower than I remember as a kid. I could be way off on this opinion, but its what I feel about clay.