I've been fighting with noise in my render. For draft renders I clamp the AO bounces in simplify, which clobbers the noise, but also the shadows. And I don't like that. So for a final render I want to turn that off. However the noise sky rockets, as you'd expect. Then I hit on the idea of removing the glass from my windows, and the noise almost vanishes.
Here's the render with glass in the windows:
And here it is with no glass in the windows:
It's not only less noisy, but brighter too. Both these are done at 128 samples with denoising intentionally turned off.
So how can I have glass in the windows and not cause the dimming of the light and the increase in noise? And why should glass cause such a massive difference?
My glass is simply a cube with this texture:
And yes, there's a portal in the window.