Each time a web server receives an HTTP request, it saves this information in a log, generally translating the user’s IP address into a domain name. This conversion (DNS lookup) is one of the Apache HTTP server’s bottlenecks and should therefore be disabled.

The 404 page should be as light as possible, or ideally nonexistent. When the browser requests a resource which does not exist (image, CSS style sheet, JavaScript file, etc.), the server replies with the 404 page, which is sometimes larger than the resource requested.
Moreover, some CMS run their content research process (in the database) in an attempt to retrieve the requested page. This results in the server code being run, the server’s database accessed and the HTML page being dynamically generated. All these processes waste CPU power, RAM and bandwidth.