There are a lot of moving parts involved in making a full-featured path tracer, and they can have wildly varying performance characteristics. Some parts such as

There are a lot of moving parts involved in making a full-featured path tracer, and they can have wildly varying performance characteristics. Some parts such as finding ray intersections in parallel and integrating them map extremely well to a GPU-like architecture, while others such as building and traversing a bounding-volume hierarchy are better suited for a traditional CPU (even though within certain constraints you can also do it on a GPU). The full rendering pipeline is much more than just finding ray-triangle intersections. It also involves material BRDFs/BSDFS (reflection/scatter properties), volumetric effects (fog, liquids, etc), motion-effects such as blurring, etc. Depending on what you are rendering, the render pipeline be vastly different from application to application. I think most production path-tracers are still primarily CPU-based, which would be because of the required flexibility.