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> while around 3/4 x faster than CPU rendering

My understanding is that for neural networks, the speedup is much more than 4x. Does anyone know why there's such a difference?

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Sure. Training neural nets is somewhat analogous to starting on the top of a mountain looking for the lowest of the low points of the valley below. But instead of being in normal 3d space you might have 1000d determining your altitude, so you can't see where you're going, and you have to iterate and check. But ultimately you just calculate the same chain of the same type of functions over and over until you've reached a pretty low point in the hypothetical valley.

OTOH, Vfx rendering involves a varying scene with moving light sources, cameras, objects, textures, and physics. Much more dynamic interactions. This is a gross simplification but I hope it helps.

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