[Plugin: WP Super Cache] WP_Super cache VERY slow load on homepage

Installed and configured wpsupercache on my site running directorypress, but cannot get the homepage to load in some browsers reliably quick. Chrome and iphone browsers consistently load up to 20 seconds for homepage. This only happens when not logged in. If logged in to the site and navigating around, site is lightning fast.

pingdom check reports site loads slower than 93% of other sites.

Can someone please let me know what can be adjusted in settings to resolve this issue?

http://wordpress.org/extend/plugins/wp-super-cache/