Spindle speed is the measure of how fast the hard drive platters of a given hard drive spin in normal operation. The spindle speed is measured in rotations per minute (RPM).
Spindle speed affects how fast (on average) the hard drive can deliver requested data to the computer. There are two separate things the hard drive has to wait for before data can be read. One is the seek time, this is the time it takes for the head to move to the requested track/cylinder. This is the figure that hard drive manufacturers like to publish. After the head has moved to the right track/cylinder, then we have to wait for the desired sector to rotate underneath the head. This is the rotational latency time, and is a direct function of the spindle speed. Faster spindle speeds = lower rotational latency times.
The combined time of seek time and rotational latency time = the access to data time, which is the figure that is given to you by hard drive benchmark programs like HD Tach.
Most standard 3.5" desktop hard drives out there today have a spindle speed of 7200 RPM. This results in a fixed average rotational latency time of 4.2 msec. These drives typically have average seek times of around 8.5 msec, giving an average access to data time of around 12.7 msec.
The Western Digital Raptor hard drives have a spindle speed of 10,000 RPM. This reduces the average rotational latency time to 3 msec. The Raptors also have smaller diameter platters, which reduces the average seek time to around 5.5 msec. This gives an average access to data time of 8.5 msec.
There are some SCSI hard drives (Seagate Cheetah) that are 15,000 RPM, with even smaller platters. Rotational latency time = 2 msec, average seek time = 3.8 msec, average access to data = 5.8 msec.