FastCGI provides a high-performance alternative to the Common Gateway Interface (CGI), a standard way of interfacing external applications with Web servers that has been supported as part of the IIS feature-set since the very first release.

CGI programs are executables launched by the web server for each request in order to process the request and generate dynamic responses that are sent back to the client. Because many of these frameworks do not support multi-threaded execution, CGI enables them to execute reliably on IIS by executing exactly one request per process. Unfortunately, it provides poor performance due to the high cost of starting and shutting down a process for each request

FastCGI overcomes this performance penalty by re-using CGI processes to service subsequent requests, while continuing to ensure single request concurrency.

Most applications built for IIS take advantage of the native, multi-threaded extensibility model of IIS. Many popular applications, particularly those written or originally designed for Linux, are not multi-threaded, and instead take a multi-process approach to concurrency. While the PHP engine itself is multi-thread capable, many of the popular PHP extensions are not, requiring a single concurrent request guarantee to operate reliably. This forces the use of CGI and results in poor performance on the Windows platform. FastCGI helps these application frameworks to achieve improved performance on Windows over CGI, while allowing stable operation in production environments.

Benchmark tests across multiple PHP applications have shown a 20-30% performance increase when using FastCGI compared to the same applications utilising ISAPI based PHP. This coupled with the many performance enhancements of IIS 8.5 allows PHP applications to perform as fast if not faster on Windows 2012 R2 compared to a Linux hosting environment