For simple installations with modest usage levels, the efficiency of scripts is of little interest. But where script use is heavy and many dynamic pages are built on the fly, the resources used by scripts and the time taken for them to execute are critical. Relatively few Web servers are dedicated to static HTML files.
Most organisations use custom programming to add functionality to their Web sites.
But where script use is heavy, the resources used by scripts and the time taken for them to execute are critical to the success of the site. Of course, all concepts in computing are compromises, and so the cost of this simplicity is performance - CGI simply doesn't run efficiently, for a number of reasons. First, each time a script runs, the server has to create a process, load the code into the process's memory segment, execute the code, then destroy the process and reallocate its associated memory. Think how long it takes to load a normal application program into memory on your desktop machine. To service a GET request requires three environment variable lookups.