WP Super Cache increasing CPU cycles, Rackspace Cloud

Hi: our wordpress sites run the Rackspace cloud and our CPU usage for the past few months had been around 30% of capacity with page load times that we thought could be much faster. We added WP Super Cache, thinking it would both help load pages faster as well as reduce CPU usage. After adjusting settings for Rackspace per their instructions, we can see that the plugin is working, and pages load somewhat faster. But alas, CPU cycles have not decreased, they have increased about +50% month-over-month (with no corresponding increase in traffic). We are using the “PHP caching” option and have banner ads that help pay for the hosting costs.

Has anyone been successful in REDUCING CPU cycles with WP Super Cache? I did a search in forums and found many other users experiencing rise, not reduction, in CPU consumption. Shouldn’t Cache be a net positive? What settings do you recommend we try? Most content can be pre-loaded (made static for up to 12 hours between refreshes), we just need to ensure that ads are still generated dynamically.

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