And the biggest, most expensive scenes are the ones where GPUs are only marginally faster. It's only very recently that GPU ray tracing could match the best CPU-based ray tracing code, and even though it has surpassed it, it's not by much, not enough to throw out all the old code and start fresh with buggy fragile code for GPUs.
Ray tracing is very incoherent (each ray can go a different direction, intersect different objects, shade different materials, access different textures), and so this access pattern degrades GPU performance very severely. GPUs are great at highly coherent work (doing the same things to lots of data at once). Don't believe the hype, ray tracing with GPUs is not an obvious win over CPU.
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