You only get those performance gains if you've got the cache and memory bandwidth - which is often very lacking. E.g. AVX (8-wide) was added for SandyBridge, b

You only get those performance gains if you've got the cache and memory bandwidth - which is often very lacking. E.g. AVX (8-wide) was added for SandyBridge, but it wasn't always that usable until Intel doubled the cache bandwidth with Ivy Bridge. With ray tracing the increase in performance going from SSE (4-wide) to AVX (8-wide) for BVH intersection was only ~25-30% - instead of the theoretical 100% increase). You're generally limited by memory bandwidth.