mod_pagespeed caches resources to disk. If you are running on a heavily loaded server or a VPS, disk IO is a premium.
So we should really cache to memory, its faster, cheaper and easy to set up.
Put this in /etc/fstab, please change the uid & guid to the appropiate user & group of your apache webserver, and the path to your needs. Feel free to change the size (here it is 256Mb) and set to use the httpd user.
tmpfs /var/www/mod_pagespeed/cache tmpfs size=256m,mode=0775,uid=httpd,gid=httpd 0 0
Save it, and after that mount the tmpfs:
/bin/mount tmpfs /var/mod_pagespeed/cache -i -t tmpfs
mod_pagespeed will now be even faster!