As complicated as the equations are for rendering, the equation that best describes the struggle when rendering is a simple one. It always seems to be quality over time. Getting nice and clean renders in a timely manner are always in the teaks to the render engine. This is especially true for physically plausible rendering and global illumination, or GI.
Understanding how render engines deal with samples and working with unified sampling in the render engine certainly helps. The other component is to understand global illumination at its core, and optimize that for memory, and less noise.
Concept Artist and Matte Painter, Saul Espinosa looks into GI engines, optimizations, and getting noise at a manageable level for production work. This latest tutorial should be the companion to a previous one, that covers unified sampling within Redshift renderer.