https://github.com/raitechnology/raikv/blob/master/graph/mt_...
The CPU in this case is a Threadripper 3970x, 32 cores, 64 SMT.
My experience is this: When the L3 cache is effective, then the memory latency hiding via memory prefetch works well across SMT threads. If the hashtable load requires a chain walk, the SMT latency hiding is less effective because the calculated prefetch location is not the actual hit. I couldn't get prefetching multiple slots as the load increased to be as effective as prefetching a single slot.
As you say, the ray tracer did a lot of cache missing , interspersed with a fair bit of calculations. I'm guessing this is close to the ideal workload, as far as non-synthetic benchmarks go.