Yeah, you want 100% power. Slow cooking a render doesn't make it better. It just takes longer.

Leaving CPU cores idle does not represent any sort of improvement of resources. Using less than 100% of memory is good. Using less than 100% of CPU is not good. It's like boiling a pot of water with the stove set to low.

Anwyays, it is true that PPro can fail to use 100% because it only divides the render into 4 pieces, one piece per core, and once that core finishes, it  does not go to help on another piece.

However, what the OP describes is not that. You don't get 1 to 5% CPU use because of quadrant allocation. You get 25%, 50%, or 75%, not some fraction much less than 25%. There is a bug in PPro. I never put my finger on it, but I have scenes involving reflections and large texture maps that do this. They do not do this in P8 or PPro 2010. I suspect it is due to excessive disk thrashing, resulting in the CPU having nothing to do while waiting for texture data to be swapped back into memory. I think it was related to the optimization introduced in P7 to make sure that textures use a constant and limited amount of memory, no matter how big they actually are or how many there are.

I usually observed this when I loaded a 10K by 5K image on  my environment sphere. This does not happen with a significantly smaller image.

I don't know what to do about it, other than find some giant textures you don't really need to be giants, and use reduced versions.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)