The performance of a computer system depends upon how the system uses and allocates its resources. Monitor your system's performance regularly so that you know how it behaves under normal conditions. You should have a good idea of what to expect, and be able to recognize a problem when it occurs.
System resources that affect performance are:
The CPU processes instructions by fetching instructions from memory and executing them.
I/O devices transfer information into and out of the computer. Such a device could be a terminal and keyboard, a disk drive, or a printer.
Physical (or main) memory is the amount of random access memory (RAM) on the system.
Chapter 3, Monitoring System Performance describes the tools that display statistics about the system's activity and performance.