In the Windows Task Manager, the rendering process is named fbxooprender.exe. When you render an image, the rendering process will use up to 4 CPUs. If other processes are also running, the rendering process releases some of the existing capacity to support those processes.
Before rendering an image, turn off active screen savers, and shut down any non-essential processes that use processor power. (For example, some web pages that use Flash graphics may slow down the rendering process.) Doing so makes more CPU capacity available to the rendering process and can reduce render time.
While rendering an image, you may want to use the Windows Task Manager to monitor processes. If fbxooprender.exe is not using close to 99% of processor power, other active processes may be interfering with the rendering process. Shut down non-essential tasks to make more processor power available for the rendering process.