Ambient occlusion is quite a helpful effect in easily achieving some realism. But the default render layer occlusion setup is somewhat render intensive. And when you’re doing lots of render tests, well you just want to see it quickly.
So we can speed up the render process by instead using Final Gather to computer the occlusion and then use the mib_fg_occlusion utility shader to make it grayscale. Then by adjusting the Final Gather render settings we can affect the render times. This is really helpful if you are doing a test render where the render quality is not paramount, but later you can increase the settings for the final render.
To the right is a tutorial of how to get the mib_fg_occlusion node setup within a surface shader. You must use mental ray since that is what Final Gather requires.