What is the "CGI Overhead", and should I be worried about it?

The CGI Overhead is a consequence of HTTP being a stateless protocol. This means that a CGI process must be initialised for every "hit" from a browser. In the first instance, this usually means the server forking a new process. This in itself is a modest overhead, but it can become important on a heavily-used server if the number of processes grows to problem levels.