It's important to remember the renderer gets exponentially slower with more resources used. For example rendering a single figure may take 10 minutes, but rendering two may take 30 minutes and rendering 8 may take 2 hours. So in total it's faster to do 8 renders of a single figure than it is to do a single render of 8 figures.
Of course as others have mentioned, if you do this overnight or while you're at work and have time to kill then no big deal. But if you're waiting then it matters.
This tool is very powerful because not only does it create an iray section plane node to cut up your scenes, it uses scripts to block the light bleed you typically get from doing that manually. I cannot recommend it enough. At the very lease use it to eliminate everything your camera doesn't see.
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This is also a huge help
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as is this
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