As HTTP is a relatively stateless protocol, each request generates an 
authentication attempt. While this is cheap when authenticating against dbm or 
htpasswd files, it can get very expensive when running against a Windows 
domain controller, ACE server, or other networked database, particularly on 
image-rich pages and FancyIndexes from mod_autoindex.

mod_authenticache provides a secure method of caching credentials in order to 
more efficiently serve in these situations.

WWW: http://killa.net/infosec/mod_authenticache/