Describes in detail the Process Priority Level setting.
Process Priority Level
Changing the value of this setting may cause performance issues. (See Scheduling Priorities on MSDN for more information.)
The process priority level determines the priority of the Robocopy process relative to other processes running at the same time. Windows uses this priority level to decide which programs should get the most CPU time.
Increasing the priority level will cause the backup operation to complete more quickly, but will also cause the computer to run more slowly while the files are being copied. Decreasing the priority level will cause the backup operation to take longer to complete, but will have less impact on system performance.
The recommended default value is BelowNormal which gives priority to any other programs that you may be using without putting Robocopy completely on hold.