Sysstat is a collection of performance monitoring utilities for Linux that includes mpstat, pidstat, iostat, and sar, and together they give you a real-time and historical view of everything your system is doing.
Most sysadmins start with top command when something looks wrong, such as high CPU usage, system lag, or load spikes. It provides a quick snapshot, but it also has its own limitation, for example it only shows the current state and does not explain what is causing the problem over time.
In real Linux performance issues, CPU is rarely the only factor, but bottlenecks usually come from multiple system areas working together: