IIS was and still is CGI capable, but starting a process on Windows is a very heavy weight thing (compared to starting a process on linux), so it doesn't scale

IIS was and still is CGI capable, but starting a process on Windows is a very heavy weight thing (compared to starting a process on linux), so it doesn't scale well and a noticeable initial delay whenever it executes. To solve that MS made ISAPI, which runs inside the webserver similar to an Apache httpd module.