The extra delay comes from resistance and capacitance in the circuit, which makes electrical signals slow down (this is the ELI5 version). DRAM cells and long

The extra delay comes from resistance and capacitance in the circuit, which makes electrical signals slow down (this is the ELI5 version). DRAM cells and long wires in the DRAM array are big capacitors and they are driven by circuits with fairly weak output (the resistance). Device physics makes these resistances and capacitances extremely hard to reduce. That is why on-device latency for DRAM has held fairly steady at ~5 ns for a very long time.