I thought I’d share a few thoughts on “optimizing” reflectance here. I haven’t done any benchmarks but the differences are generally noticeable without a stopwatch.
Use the physical renderer and the adaptive sampler. The quality to speed ratio is just higher than the standard renderer.
Increase Blurriness Subdivisions if you’re using rough reflections. These are targeted subdivisions that will improve the quality of the blurry reflections without adding more subdivision samples to the rest of the scene (where you likely don’t need them).
Don’t use diffuse reflectance. This feature essentially replaces GI if you choose to use it, but it doesn’t have the same level of control over the quality/speed. It’s essentially like running QMC GI with a large diffuse depth (both in terms of quality and render speed). Instead stick to using GI and the Color channel.
Consider enabling Add Grain (Slow) on your Area Lights if using the physical renderer. Not directly related to reflectance but something I’ve discovered in use. I’d never used this before since it warns (Slow) directly in the attribute. However, it’s not slow when using the physical renderer and it improves the lighting dramatically for large or complex (e.g. spline or object based) area lights. If you want to reduce the grain you can increase the Shadow Subdivisions to do that (the Samples attribute has no effect with Add Grain enabled in the physical renderer).
Those are some working tips I’ve developed. Anyone else have some suggestions?