Performance is slow on a multiprocessor computer that is running Windows Server 2008 or Windows Vista
You encounter one of the following issues on a multiprocessor computer that is running Windows Server 2008 or Windows Vista:
Performance is slow if the number of processors is greater than 64.
Only half of the processors are started. For example, 12 processors are started on a computer that has 24 processors.
For more information on this issue, including potential causes, workarounds, and resolutions, see: Microsoft KB Article KB969468.