The problem with regular old CGI is that for every request the web server gets it has to run some binary file and setup a new thread for that every single time.

The problem with regular old CGI is that for every request the web server gets it has to run some binary file and setup a new thread for that every single time. Also, that's ignoring the filesystem overhead which isn't negligible even if it's already cached. You're basically turning your web server into a glorified shell script and even though all of your CGI scripts are compiled it's still horribly inefficient. FastCGI on the other hand is basically like giving the web server a socket to a persistent process that runs the associated code in an existing thread instead of having to spawn a new one on every request. Basically FastCGI is like having another server just for your dynamic content while the web server sits in front of it and takes care of hosting the static content and deferring to the FastCGI server when appropriate.