RAM ~100 ns, SSD ~100 µs, spinning disk ~10 ms — each rung roughly 100× slower.
Six orders of magnitude between memory and disk is why caching dominates systems design. One disk seek costs ~100,000 RAM reads.
The rungs have moved. NVMe is nearer 20 µs than 100, and Optane closed the gap to RAM far further than the ladder allows. Queue depth matters more than the medium in many workloads: a spinning disk streaming sequentially can beat an SSD serving random 4 K reads.
RAM ~100 ns, SSD ~100 µs, spinning disk ~10 ms — each rung roughly 100× slower. Six orders of magnitude between memory and disk is why caching dominates systems design.