Is there a speed difference between WSGI and FCGI?
They are two different things. WSGI is a Python specific interface for writing web applications. There are wrappers for about any web server protocol to provide the WSGI interface. FastCGI (FCGI) is one of such web server protocols. So, WSGI is an abstraction layer, while CGI / FastCGI / mod_python are how the actual web servers talk to the application. Some code has to translate the native interface to WSGI (there is a CGI module in wsgiref, there is flup for FastCGI, etc.). There is also mod_wsgi for Apache, which does the translation directly in an Apache module, so you don't need any Python wrapper.
From the web I've gleaned that WSGI is a CGI for python web development/frameworks. FCGI seems to be a more generalised gateway for a variety of languages. Don't know the performance difference between the two in reference to the languages python and C/++.