Speed of light delay due to trace length really is not that important to DRAM. Adding ~6cm to the round-trip path would add about 0.3ns to DRAM latency, which i

Speed of light delay due to trace length really is not that important to DRAM. Adding ~6cm to the round-trip path would add about 0.3ns to DRAM latency, which is already over 30ns. So we're looking at less than 1% difference between on-package vs on the motherboard. This is a much less important concern than signal integrity and drive strength necessary to maintain bandwidth when moving DRAM further from the CPU. Your argument would be much closer to relevant if we were discussing SRAM caches.