With "Branched Pathtracing", you could set the "Anti-Aliasing" Samples(*) to for example20 (this is the number in the "Render Samples" field and the "Diffuse Sub Samples" to 4 in order to get 80 Samples per pixel for rays hitting a diffuse surface at the first bounce after beeing sent out from the camera. The same number could be achieved with just 1 "Anti-Aliasing" Sample and 80 "Diffuse" Samples. What is faster and/or gives the best render results given a scene with mainly diffuse objects. Of course, for a complex scene with many different surface types, it can be necessary with respect to the rendering time that you need to do the adjustment in the "Sub Samples" section.
(*) that is the number of "Camera Rays" sent out per pixel in order to calculate an average light value which approximates the real world lighting more or less accurate depending on the number of samples choosen)
The more specific you get to the materials in your scene, the more optimized the render will be. So if you have no glossy surfaces, I believe 1 AA sample and 80 Diffuse samples would work better than 20 AA samples and 4 Diffuse samples. Exactly how much faster, though? I have no idea.