OK, back to the original question. Here are a few concrete items I can point to as render time hogs, having been spanked by a few 15 hour render times in DS3.
Hair and feathers with transmaps. Transmaps eat cpus for breakfast. This is especially true in close-up renders.
Raytraced shadows eat up even more cpu. Breakfast and lunch! For some scenes, deep shadow maps are quite sufficient, IMO. Raytraced shadows + transmapped hair = 12+ hours.
Reflections also increase render times. In the scene with the 15 hr render time, I had raytraced shadows, transmapped hair and feathers, AND a champagne glass with reflection and refraction effects. Dialing raytrace bounces up to 6+ makes renders happen in geologic time. This is why a lot of people use reflection maps to fake reflection effects rather than use reflective surfaces and grow old waiting for the render.
My 15 hr render: http://slimerjspud.deviantart.com/gallery/#/d4eap0m Note the refraction effects on the glass and the shadows from the feathers. For an indoor scene, DSM shadows would have done almost as well.
I have run a few Reality renders (remote Luxrender actually), and I won’t go into more details about how that goes as I’m a newbie there. Because everything is so much more “real” in an unbiased render with no phony constructs like distant lights, it naturally takes forever.