For hard disk drives, disk access time is determined by a sum of the spin-up time, seek time, rotational latency, and transfer time.
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Rotational latency is incurred because the desired disk sector may not be directly under the head when data transfer is requested.
From the en.wikipedia.org
Traditional HDD benchmarks are focused on finding the performance aspects where they are weak, such as rotational latency time and seek time.
From the en.wikipedia.org
As a result, in order to hide the initial seek time and rotational latency, data are transferred to and from disks in large contiguous blocks.
From the en.wikipedia.org
Average rotational latency is shown in the table below, based on the statistical relation that the average latency in milliseconds for such a drive is one-half the rotational period.