Hello Glenn
The performance setting disables several power saving features. The CPU and memory run at higher voltages, and do not downclock during idle times. Your benchmarks may be a little skewed as there is a brief amount of time required to increase speeds on the CPU and memory from idle. This can cause benchmarks to report inaccurately if they are short tests.
You can read more about it in this white paper: http://en.community.dell.com/techcenter/extras/m/white_papers/20248740.aspx
Thanks