Might be dirty and thermally throttling. Considering the age of the OS/laptop, you should probably consider both physically cleaning it, which should probably include opening it up and removing any dust, and reapplying thermal compound, also a clean reinstall of Windows. If you're using generic drivers, don't, stick with what Dell provides. You should also download HWinfo64 and monitor the temps and usage, and make a habit out of knowing how your laptop works.