New version! - 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. However, I took a different approach as I needed to develop the backing code anyway. More on my little utility later.
According to public documentation on the feature, unpark time is near instantaneous, but there *is* some overhead. Additionally, parking CPUs can potentially reduce core thrashing - when a thread is swapped between cores unnecessarily, causing a performance hit. This is one of its primary roles, in fact, states a document from DELL on configuring its servers.