Interesting comments from Nadal on surface speed. Basically, he inidcates surface speed is irrelevant if you're timing the ball correctly. Andre Agassi expressed similar sentiments in his book, basically saying that he didn't try to change anything about his game for grass courts --which is why he never practiced on them -- because he knew all he needed to do was adjust his timing, and he'd be fine.
http://www.tennis.com/pro-game/2014/06/grass-court-state-mind/51858/#.U6vLgfldVQ0“The problem with the players is always the same,” he said when he was asked whether the grass was too fast or too slow. “If I am playing very well, I see the grass slow. If I am playing bad, I see the grass very quick. That is the same for me and the rest of the players.”
“You arrive, for example, to a tournament in Montreal that is playing very fast. I remember 2005 I was there. I had an unbelievable feeling, I was playing great before I arrive to the tournament. I arrive to the locker room, everybody was saying the court was playing that fast. I didn’t feel the court was that fast. It’s always about the personal feeling of the people.”