Missing AO panel & increased Cycles render time in 3.5 vs. 2.93

Question

I had a simple setup made in Blender 2.93.9 (see file here) which combines 2 scenes in the compositor:

• Main scene, which renders an object in Eevee
• Shadows cast on a plane shadow catcher in Cycles

The composite result in Blender 2.93.9 looks the way I want: But when I render the same file using Blender 3.5, I get this:

The Cycles shadows are much darker. I figured this might be caused by the AO, but I can't find the AO settings for Cycles in 3.5. In 2.93 there was a specific panel in the world settings, which looked like this:

Where is this panel & how do I adjust AO settings for Cycles in 3.5 ?

The second issue I have with this scene is the render time: around 2" on my hardware using 2.93 vs. 30" on 3.5, which is 15x render time ! I can improve this a little bit by disabling the Cycles denoiser without too much loss in quality (it's only for shadows). This reduces the render time to about 8 seconds, but that is still 4x render time.

What changes brought to 3.5 cause Cycles to take so much longer ?

Answer 1

In Render Properties under Light Paths you will find a Fast GI Approximation check box and dropdown menu:

The other option is to use the Ambient Occlusion node in the Shader Editor with the World Shader selected.

Answer 2

So, after some extensive research (see this article), I found that the "Ambient Occlusion" panel from 2.93 that I was looking for might not be actual AO, but more like global illumination which adds on top of the Cycles ray tracer. Of course, this is not physically accurate, which might explain why it was removed on later Blender versions... But what if people like me were using this specifically, as some kind of effect ?

For now, the best workaround I found to get this to work on 3.5 is to increase the World Background strength from 0 to about 1.1, to simulate the loss of the so-called AO. I don't get exactly the same result as on 2.93, but at least the shadows are not that dark. As @John Eason pointed out, it can also be done using an AO node in the World Shader, the results are very similar:

However, neither of these 2 solutions are a perfect replacement for the old so-called AO panel, especially if you render the whole scene in Cycles:

Scene in 2.93 with 0 strength background, but so-called AO panel on. This is what I want.

Scene in 3.5 with 1.1 strength background, or AO node in World Shader. Current workaround, not the same, but it looks OK.

And there's still the issue of render time...