It still blows my mind that he doesn't do well on clay and these slower surfaces. His game to me seems like it is built for those surfaces. But in th end he has won the Big W and the US Open. Go figure.
Yeah, you would think with his speed and consistency he would kill on clay but on the other hand, I can see why he doesn't do all that great on clay. He does surprisingly well on faster surface with his groundcourt game because it makes his serve more potent, and he doesn't hit with a lot of spin (compared to true clay courters). On faster courts his relatively flat strokes penetrate a lot more than those of clay courters, but on clay his flat strokes mean less consistency. Less consistency because his opponents get one or two, even three or four more balls than they would on a faster surface, and he's not THAT consistent.
Also, even though he's fast as heck, he's not really a natural mover on clay. He can't move with the same economy of motion of somebody who can slide on clay without effort, which means later into the draw he'll be more tired.
I do think the US Open considers what type of surface will benefit Andy and Agassi, or Sampras when he was playing. He might have a bit of a point on that one.