. . Clues, like swing path and racket face orientation, can often be subtle and may be picked up more on a subconscious level rather than conscious one. If you play against a particular server enough, you may become better at picking up these subtle clues.
. . Auditory clues are very important as well. Even tho' sound does not travel as fast as light, with tennis court distances, there is almost no difference. However, it takes longer for the brain process visual information that auditory info -- we react to sound much quicker than we react to visual stimulae. A simple reaction time for sound can be 50 ms quicker than a simple RT for visual info. In some cases this will be 20% quicker (or better). We can pick up cues about ball speed and spin before the info to our eyes is registered by the brain.