I realise so many people have asked this question so many times, but I just can't see what I'm doing wrong here.
I'm on Blender 2.83.2. I have a simple scene with a room and some objects, some with glossy shaders. There are 3 point lights on the ceiling providing plenty of direct and indirect light, so shadows are soft and mostly diffused, and in most cases there are very few shadows due to the generous slather of light being dumped into the shot from 6 point lights on the ceiling.
This is a portion of the shot rendered in Cycles without denoising for reference:
And this is the full frame with denoising creating what you expect... a shot that looks like someone smeared my camera lens with a potato.
These are my cycles settings, which are set to default except for indirect clamp being set to 10 as part of my own troubleshooting. I've tried samples up to 512, and there was virtually no difference from the results I got with 256 samples. So I suspect I'm doing something else to create this noise beyond just being stingy with compute power:
I used to think Cycles only struggled when lighting was low, so I was surprised when I first switched from Eevee to see how it would look.
Any help is greatly appreciated. Thanks.