FastCGI is a protocol developed to enhance the performance of Common Gateway Interface (CGI) scripts, particularly in high-traffic web environments. Compared to traditional CGI, FastCGI provides a more efficient way for web servers to interact with external applications or scripts to generate dynamic content.

Essentially, FastCGI works by using a process pool to manage the execution of scripts. Unlike CGI, where a new process is started for each request, FastCGI keeps a group of processes running persistently, waiting for requests. This reduces the overhead costs of starting and terminating processes and leads to an overall faster and more efficient processing of web requests.

FastCGI also provides the ability to transfer data efficiently between the web server and external applications, further enhancing performance. Additionally, FastCGI supports features like multiplexing, where multiple requests can be processed simultaneously over a single connection, improving scalability.

Due to its performance advantages, FastCGI is often used in conjunction with web servers such as Apache, Nginx, and Lighttpd to efficiently serve dynamic web content. It is a key technology in web development, especially for high-traffic websites and web applications.