ParkControl is a handy and reliable utility designed to allow you to adjust CPU Core Parking for one or more power profiles.
CPU Parking is a feature of multi-core/SMP systems that disables cores when the system load is low. This decreases energy consumption and reduces heat dissipation.
By default, Windows provided no direct way to adjust this via the user interface, as they likely thought everyone would want to keep this enabled. With some tweaks, you can make it come back to the Power Profile settings.