That is usually a sign for to few samples for reflections on rough/diffuse surfaces, so the first step would be to increase the samples for raytraced reflections.
Reflection noise.
Not sure, if there are many ways of optimising reflections, other than balancing the samples, the roughness (there is a max roughness setting in the Post process, which makes it blend or slowly switch to skylight i guess), and that raytracing quality node in materials.
Maybe this here helps, the reflection optimisation starts at 32:48
