It's a correlation != causation fallacy. The ace percentages being down might be indicative of slower conditions but it also might be indicative of other factors such as match-ups; fitness/form/serving performances; players standing further back to return and at least touching the ball more regularly; and so forth.
Also, I wish we didn't have this tendency to say "the surface is slow" when conditions being slow can be as a result of any combination of atmospheric conditions, the balls and the surface itself.