Thread and Fiber Execution
Microsoft Windows uses a numeric priority system that ranges from 1 through 31 to schedule threads for execution. Zero is reserved for operating system use. When several threads are waiting to execute, Windows dispatches the thread with the highest priority.
By default, each instance of SQL Server is a priority of 7, which is referred to as the normal priority. This default gives SQL Server threads a high enough priority to obtain sufficient CPU resources without adversely affecting other applications.
The priority boost configuration option can be used to increase the priority of the threads from an instance of SQL Server to 13. This is referred to as high priority. This setting gives SQL Server threads a higher priority than most other applications. Thus, SQL Server threads will generally be dispatched whenever they are ready to run and will not be pre-empted by threads from other applications. This can improve performance when a server is running only instances of SQL Server and no other applications. However, if a memory-intensive operation occurs in SQL Server, however, other applications are not likely to have a high-enough priority to pre-empt the SQL Server thread.
If you are running multiple instances of SQL Server on a computer, and turn on priority boost for only some of the instances, the performance of any instances running at normal priority can be adversely affected. Also, the performance of other applications and components on the server can decline if priority boost is turned on. Therefore, it should only be used under tightly controlled conditions.