We're now well out of my knowledge base, but your comment certainly makes things interesting. If they have similar NUMA latencies (big if!), I guess maybe the edge would go to whichever chip had higher NUMA bandwidth? Just sort of thinking out loud...

Or perhaps it has absolutely nothing to do with memory speeds, but I can't think of what else would have such a dramatic effect. I'd love to see the same test with a single Xeon.

I think it’s the other way around: four 8-core processors

It's four dice with two groups of cores of four each connected by a callosum.

In NUMA-speak, each group is (probably) a node with neighbouring nodes having a lower distance.