Edit menu - General Preferences - Render - change slider for number of threads (I think).

IIRC the Firefly sliders for Irradiance Caching and Indirect Light Quality work together. Higher ILQ = more rays; high IC = less guesswork. The higher they are the slower it goes. So, if one is higher the other can be lower. The more guessing IDL does the more blotchy artefacts but the faster it renders. It is a constant trade-off between quality and speed. The pre-pass with red dots is a good indicator - the more dots and the more densely packed, the higher the quality but the slower it will render.

As Richard60 says, you can turn off IC and brute force it, but the render speed becomes impractically slow. IDL does have occlusion built-in and it is better than AO because it takes account of light direction. If you render without IDL completely you could try playing with the Shadow Min Bias setting on the lights (in the Properties tab) - setting it lower than default can help with fine shadows but too low and polygons will self-shadow.

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