You only need to do fun scans at intervals of maybe once a week, I think. Does Norton allow you to choose the time to do it- e.g. Sundays at 4am so it doesn't inconvenience you
I believe the S200 has a power save feature that is accessed via the Fn + F2 key combination. Maybe worth checking as this will slow the processor speed to save power if active. Also, make sure that the main power adapter is plugged in as this could also reduce CPU speed depending on the power setting plan.