It's SRAM vs DRAM not the speed of light. Go take a look a DRAM CAS values as clock speeds have increased. The ns/fetch has stayed pretty static(~10ns) since DD

It's SRAM vs DRAM not the speed of light. Go take a look a DRAM CAS values as clock speeds have increased. The ns/fetch has stayed pretty static(~10ns) since DDR first showed up. There's been slight improvements but nothing like the order of magnitudes you seen in other areas. If you want to pay the power and cost of SRAM it can be done but it doesn't usually pencil out beyond what you see in caches today.