There is a lot of overhead as far as windows is concerned managing a single threaded process into a dual core/processor situation. It often outweighs the benefits of the additional processing power. Try opening task manager, selecting the processes tab, right click the solidworks process, choose "set affinity" and de-select one of your two cores (does not matter which) . This is a session only setting, so next time solidworks is started it will revert back to using both cores. I'm willing to bet that your benchmark times improve.
I usually leave mine set up to use both cores. But when I am doing something that I know is going to be processor intensive, I'll switch it over to single for speed. There are utilities available that will automatically run a selected processor in single core mode, should you prefer it that way all the time.