I think Castafiore made an interesting (and valid) point that Nadal likes a surface where the topspin works, but not necessarily a slow one. I have a bit of difficulty to get my tennis experience and my physics knowledge to agree how these things work. But let me give it a try, and then maybe some person can help to fill in missing pieces.
What is it that makes the conditions fast? Apparently a bit of altitude like in Madrid and the thin air helps making the conditions fast, as the air friction diminishes with altitude. Also the choice of balls can be important (a more or less elastic bounce from the racket is of course rather crucial when it comes to speed, and the weight of the ball influences the anount of power needed). But what does the surface do? Clearly a low bounce forces the player to hit the ball earlier after the bounce (as otherwise they have to hit below the optimal strike zone), which increases the speed of the play. Also the friction between the ball and the surface affects the speed, but it can be in both directions. A high friction can slow down a ball with little rotation (as the high friction then helps to convert translational energy to rotational energy), but it can also help to speed up a ball with lots of spin after the bounce(a high friction helps to converte a high rotation energy partly to translational energy). For this later effect to work a rather high minimium spin is required, but my guess is that most players with heavy topsin typically is well above this limit.
Nadal doesn't like heavy and slow conditions like after rain. But why is it so? The possibility to hit hard and flat is not so much decreased, even if the balls now are heavier. You may need a bit more strength to hit hard, but it can be done. But the heavy balls make it really difficult to give a lot of the spin to the balls. Maybe the friction between the racket and the balls tend to decrease when the balls are a bit wet, or maybe it just requires to much physical strength to do it.
If the friction between the surface and the ball is high, a heavy topspin will transfer some pace to the ball after the bounce. But still this effect typically helps to slow down how the courts play. Usually if the players don't want to take the ball on the rise, the high bounces due to the spin forces the player to take the ball way back in the court, leading to a slow play. The reason play is slow on clay courts is not due to lack of speed in the different strokes used, but due to players taking the ball way back in the court a long time after the bounce.
Where does this leave us? Most certainly Nadal wants his topspin to work. To some extent this is the same conditions that makes surfaces slow, and that is why Nadal is considered as a player that excel in slow conditions. But the conditions after rain is a bit of a counter example, illustrating the point that Castafiore made, that the mechanism that favors Nadal is the effectiveness of topspin, that is only partially related to the speed.
Rain wets the air.
Moisture is the name I believe.
Nadal's best is on dry,fast clay and warm air.
Slow heavy wet clay and wet air spoils the playing surface and balls.
Balls do not get up on mud and take no spin.
ugly conditions prevent the best player from winning in any surface.