SPDY is a new networking protocol used for speeding up the web. It combines with http protocol with several speed related features that can reduce page load time. It uses stream multiplexing and header compression to improve speed.
In order to use SPDY, a web server and a browser that both support SPDY is needed. Google chrome and newer versions of Firefox support SPDY protocol. In Apache HTTPD server mod_spdy module add support to SPDY protocol. SPDY protocol uses HTTPS so site need to serve content over HTTPS in order to benefit from mod_spdy.mod_spdy serves requests like Apache worker MPM. It serves requests using an internal thread pool which interact badly with non-thread-safe Apache modules. In order to server php over mod_spdy use mod_fcgid rather than mod_php.This is because as some PHP libraries are not thread safe using mod_fcgid will run PHP in a separate process thus avoiding thread safety issues.