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The CPU rendering mode is actually meant to be used by people using KeyShot on systems with lower-end videocards, specially laptops (I was testing KeyShot 9 on a MacBook Pro 2013). If you have a powerful videocard, chances are you would like to take advantage of it and use the GPU rendering mode. The story is completely different if you are using the GPU renderer. Because the GPU usage doesn't really affect your computer performance (it might affect display a tiny bit, but the OS in general will not run slower if your GPU is running at top performance). If you have ever rendered big images or animations before, you may have experienced this.
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I wouldn't say this is a problem with the software, though. Personally, I would advise to not use all of them when working with KeyShot, because that can make your computer hard to use since it will work very slow, resulting in trouble adjusting the camera position and pretty much any other operation. You can, however, define how many cores you want to use. If you are using your CPU to render, you will notice the computer performance can drop drastically, because, by default, the software is using all the CPU cores available. When you open KeyShot, you are greeted with a welcome screen that links you to tutorials, news, and also some sample projects that you can use to check out KeyShot's capabilities.
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