I found it. It may have something that is helpful:
Another thing to consider is whether the plane / mesh surface being reflected on is scaled too high to generate non-blurred results. Materials get stretched on mesh surfaces that are stretched or expanded beyond their original size. To combat it, it’s dependent on the context of the scene and how the meshes were created with UV unwrapping and wrapping and such. However, in the picture posted, all those planes (meshes) are virtually the same size, and one is clear while others are blurry based on the roughness setting. So, you could approach it in a way where when you have altered to one of the tutorial’s suggested settings, then tweak roughness within 0.05 to 0.2 or so to see how it affects the blurriness…if it corrects things. One of the main issues I see in the whole thing is the skylight picking up the color / roughness on nearby surfaces, such as the floor which is rough looking, and using it in the reprojection of lighting, which then is injected in the reflections, I think.