When using long complex pixel shaders its important to minimize the number of pixels rendered so that total shader compute time stays within reason.
One way of doing this is to do an initial Z only pass (meaning no color buffer writes or pixel shader execution) for all objects in the scene. Then do a second pass with shading turned on. The early Z test will then eliminate work on all non visible pixels. This approach should not be used with simple single texture shaders since the cost of two passes can easily exceed the relatively low cost of rendering with a simple shader.
