Er, neither has a GPU. The i9-7900X has the benefit of generation of tuning and runs at a slower base clock (3.3 vs 3.6 GHz for the 6850K). Slightly smaller l3, and a much larger but also higher latency L2 helps as well.
I suspect moving from a ring bus to a mesh helps as well.
I suspect moving from a ring bus to a mesh helps as well.