Perfecto, i'm not sure how much the clipping plane affect render times but what I do know is that although the clipping planes are imaginary Maya uses them to calculate depth information so obviously the larger the distance between the planes the more calculating it would have to do.
Be aware that Auto render clipping planes is on by default when you use the Software renderer and only works with that, if you want to use say the Hardware renderer or Mental Ray then Maya uses the values that you have manually set or are there by default on the near and far clipping planes.