Ray tracing is the most expensive because buy wow classic gold instead of approximating how bright an object will be due to the distance to the light source (as in blinn phong, for example), you following the theoretical path a photon could take out of... moreRay tracing is the most expensive because buy wow classic gold instead of approximating how bright an object will be due to the distance to the light source (as in blinn phong, for example), you following the theoretical path a photon could take out of the light source, to see what color it would be when it hit your camera (to do this, it actually sometimes easier to trace a path backwards from the camera)it kind of like how general relativity is a "step up" from newton gravity. You include more variables and calculations, and you get more accurate information in the end.performance vs appearance is a hot topic, but a lot of physically based rendering engines have shaders that are optimized up the wazoo to balance performance and appearance. And, as a plus side, ease of artist use (for example, when they texturing their models, the parameters seem intuitive to them like "surface roughness" instead of something like "specularity coefficient," etc) is really easy with physically based rendering, because... less
