At 5ghz, the signals travel only 4 cm each cycle (signal speed in copper is around 60% lightspeed).

Moving stuff further away means waiting multiple cycles for a reply.

Random article:

> Power10 chip .. using the DDR4 buffer chip from MicroChip and only adding a mere 10 nanoseconds to memory latency.

At 5GHz that would be 50 cycles or 1m round-trip. The delay is in processing - bridging logical and technical protocols, resynchronization, fanout, probably even a cache layer.