The Intel Core i9-12900K is the objective winner of this comparison, securing victory in ten out of fourteen measured specifications. Its most significant technical advantage over the Intel Core i5-8400 is its L2 cache, which is 833% larger. This substantial increase in cache capacity provides the primary performance edge for the newer processor, establishing it as the superior component across the majority of the evaluated metrics.

Users prioritizing raw processing capacity and advanced architecture should select the Intel Core i9-12900K for its extensive cache advantage. Conversely, the Intel Core i5-8400 remains a viable option for those specifically seeking a lower Max Turbo Power, as it holds a 73% advantage in that category. Choosing between these two depends on whether your requirements favor the high-capacity cache of the newer model or the power efficiency profile of the older unit.