You may notice increased IO latencies that negatively impact performance on Windows Server 2008 R2 compared to earlier operating systems on the same hardware when power management is enabled. You may also notice lower CPU utilization in Task Manager and on computer with multi-core processors observe CPU's being "Parked" in Resource Monitor.
By default, Windows Server 2008 R2 installs with a "Balanced" Power Plan. The processes currently running on your computer are not requesting enough CPU processing cycles to overcome the interaction of that power management plan with your hardware and Windows is parking processors on your system in an attempt to conserve power.
Check with the OEM of your computer for their recommendations for power plan settings as they may have settings specific to your computer. You can also change your computers power plan in the Control Panel - Hardware - Power Options page to "High performance" to see if this improves performance. The High performance plan alters the core parking thresholds compared to the Balanced plan.
While Core Parking can result in improved energy efficiency and cost savings, computers in environments where high performance or availability are required may benefit from turning core parking off.