There are a few paid builds out there that have really improved on the Cycles engine also. I have a feeling Cycles X will be using many of these new improvements that appeared in these paid, standalone releases, such as e-Cycles and k-Cycles builds. The major improved features have to do with Optix and RTX optimizations as well as improved Cuda optimizations. I found e-Cycles, for example, to be about 30% faster on the exact same animated scenes when using similar settings. e-Cycles opened up a few render optimization settings to the UI that wasn't available directly with vanilla Blender builds. I heard k-Cycles is even faster and more optimized than e-Cycles, but I haven't tried that one yet.

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