Over in the Carrara forum at DAZ an incredible tip was mentioned about how to literally cut render times in half (I've tried it multiple times and it actually really does work; I have no idea why).

So you've set up your scene, using full global illumination or skylight or whatever, it's complex and filled with detail and set to render at a large size and you know it's going to take some time.

The trick is before you render the scene at full size you first render it at an incredibly tiny size (shrink it down to a 2pixel/2pixel size, for example) and it will take only a minute or two to perform calculations and render (the pic will just be a tiny dot).  The important thing is that you check the option to 'save irradiance map' before rendering (you will also have to click on the ellipses below that section and name the map that you are saving as well as designate a location where you are saving it; additionally I think it is best to select 'calculate one map for all frames').

After the tiny render is done and the irradiance map has been saved, now uncheck the 'save irradiance map' and select the 'use irradiance map' and click on the ellipses in that section to select the irradiance map you just saved.

Now render at full size, and watch how much more quickly it will render the full size complex scene.

Renders that take me 4 hours to render normally will take about 2 hours and a few extra minutes to render using this method.  It's crazy, but it works and I can't believe how exciting and useful this little tip has been for me.  :)   I used to never use full GI but now with this little trick full GI is something I can use all the time, even with really complex scenes.

I've tested this on several complex scenes and I'm getting render times nearly cut in half every single time.  Does anyone have any idea why this is working?