FastCGI cache is faster than WP Super Cache because the latter uses .htaccess and and PHP itself to route the visitor to the cache (files). So before the visitors get to a hit on a cached page, WP Super Cache has to performance some logic both in the form of .htaccess (rewrites) and PHP itself (all WP plugins must use php). Whereas FastCGI uses compiled bindings that are fast and routes traffic directly to the cache.

Now, an HTTP cache like Varnish will always be faster than the two. That’s because you’re traveling one fewer hop down the stack to get to the data, which in the case of Varnish sits at the HTTP level as an HTML object and can be served hot.

So to illlustrate:

So to get to the:

FastCGI cache: Nginx -> FastCGI -> File

WP Super Cache cache: Nginx -> FastCGI -> PHP -> File

Varnish cache: HTTP