Diffuse material looks weird/noisy

[Brian Jones]

to clean up the grain you have to go down to Layer Sampling in the reflectance controls and increase the Sampling Subdivisions to whatever it takes to clean it up. This will slow your renders by a lot. Color in reflectance is not faster it’s slower but it is a form of GI so this can be a good trade off. It depends on what you need for your final look. And whether normal GI with standard materials is faster than a reflectance workflow (that can depend on the scene). If you don’t need a GI look then use standard materials.