I left the background objects because in my render they were not much of a burden on the render. if you have lots of stuff in the background, i would suggest turning off anything that is not in the area render. just make sure you know where your shadows are falling so you get the area reanders to cover them. ya know now that i think about it im not sure you need to turn anything off after you get the background rendered. you could try rendering the background then turn everything on and area render smaller sections until you get everything else covered.

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