Path tracing is currently state-of-the-art for making photorealistic CG images because it can generate real reflections, refractions &c. by simulating something very similar to how the physical world works.
A photo (or movie) is, however, taken through a lens that distorts the image (sometimes intentionally; what is and isn't in focus in a composition is important for drawing the viewers attention).
A path tracer can already simulate this by just putting a bunch of pieces of glass shaped like the real lenses of a camera, but this is extremely computationally intensive.
This particular implementation uses a few (already known in the literature) techniques for making it faster: bidirectional tracing, selective oversampling, and thin-lens approximations.
This is an implementation of these techniques for Arnold, which is a renderer implementation used for many feature films[1].
Probably not much - the reality is that rendering depth of field in camera in production CG is a very tough sell. Even if there is no speed penalty, renders are usually so expensive that it isn't even a question of doing it in real time in compositing.
