The speed of light is roughly 30cm/ns. So accessing main memory 15cm away on a motherboard is about 0.5ns slower than cache, no matter whether the main memory is DRAM or SRAM. That's 2 clock cycles extra at 4GHz, which is a tiny fraction of the actual time (somewhere around 100ns) but not negligible. 0.5% or so is just enough that I'd say it can matter. Particularly since larger computers end up putting some of their RAM further away.