Session at DrupalCamp Wrocław 2013
On Debian, apache installed from repositories, has mod_expires turned off by defauls. This seriusly increases server load. So what does mod_expires do? It "controls the setting of the Expires HTTP header and the max-age directive of the Cache-Control HTTP header"... Let me explain. The module allows apache to inform clients (eg. You browser) about the length of time a particular asset can treated as valid.