Also, bandwidth becomes a factor as well, since the Internet is only as fast as it's slowest connection. The filtering server simply allows or disallows the URL request. An allowed site will then download apart from the filtering server. A disallowed site will not load at all. Since a server can only handle a limited number of concurrent users, more subscribers require more servers.
More servers require more maintenance and more bandwidth.
This, in turn, requires more technicians and more time. All of these factors increase overhead, as well as the risk of downtime. S4F enables a single server to handle far more concurrent users since the site requests are processed in a fraction of a second. Proxy servers can be occupied up to several minutes with a single webpage, so the quick release of the S4F server enables it to handle much more work.