Modern disk drives are fabulously fast. Seagate announced a new disk drive in the fall of 1997 that spins at 10,000 rpm and can transfer 11 megabytes in one second. However, disks are still much slower than RAM and here's why. First, the actual data transfer rate, the speed at which bits are written onto the platter surface or read from it by the read/write head, depends upon the angular speed of the platter, i.e. its rate of rotation. Since it takes more and more energy to whirl the platters faster, there are practical limits. Moreover, the bearings which anchor the platters to the disk drive frame wear out if it spins too fast.
Even so, the data transfer rate, the time it takes to read or write one full sector, is small compared to the time it takes to get to the right sector, which breaks into two parts:
The larger the platter is, the longer the rotational delay because the tracks are longer. Another anomaly is that the bit density is greater towards the center than at the rim because of the differences in lateral (sideways) velocity when the angular velocity (rate of rotation) is held constant. Modern disk drives compensate for this in various complicated ways.
Seek delay is often the longest component of the total time it takes to read or write a disk, because the stepper motor must be turned on, kept on until the read-write head moves to the correct track, and then turned off. Electric motors, like any physical device, cannot accelerate to their full operating speed instantly, so allowances must be made for the acceleration and deceleration of the motor. Seeking to a distant track is usually not as long as it would take to go to that track by doing one track seek at a time since the stepper motor, once on, can be left turned on until the read-write head nears the desired track.