Rotational latency
The wait for the target sector to spin under the head after it reaches the right track — half a revolution on average, gone entirely on SSDs.
Because any given sector can sit anywhere in the revolution, the average works out to half a turn. That is 4.17 ms at 7200 rpm, 3.0 ms at 10,000 and 2.0 ms at 15,000 — the delay falls straight off with spindle speed.
Together with seek time, rotational latency dominates a hard drive's access time; caches and reordered command execution (NCQ) hide some of it by reading sectors in the order they pass under the head. An SSD has no moving parts and therefore no rotational latency at all, which is a big part of why they feel instant.