Sometimes drives have the needed data in their buffer so the drive will not have to perform a seek to get the data. If the average seek time of the drive is 20 msec but for half of the data requests the buffer already contains the data, the effective seek time will be about half of 20 msec, or 10 msec. It takes almost no time at all (compared to the time it takes to do a seek) for a buffer to deliver its contents, so the average time required drops significantly. In this example, by one half.