System Configuration : System Setup : Configuring Pro Tools System Settings : CPU Usage Limit

CPU Usage Limit
The CPU Usage Limit setting in the Playback Engine controls the percentage of CPU resources allocated to Pro Tools host processing tasks. Used in combination with the Host Processors setting, the CPU Usage Limit setting lets you control the way Pro Tools tasks are carried out by the system.
*   
Lower CPU Usage Limit settings limit the effect of Pro Tools processing on other CPU-intensive tasks, such as screen redraws, and are useful when you are experiencing slow system response, or when running other applications at the same time as Pro Tools.
*   
Higher CPU Usage Limit settings allocate more processing power to Pro Tools, and are useful for playing back large sessions or using more RTAS plug-ins.
The maximum available CPU Usage Limit depends on the number of processors in your computer and on the number of processors you specify for host processing. This value can range from 85% for single-processor computers (except for 003, 003 Rack+, 003 Rack, Digi 002, and Digi 002 Rack, which have a limit of 99%), and 99% for multiprocessor computers (which dedicate one entire processor to Pro Tools).
On multiprocessor computers, the maximum CPU Usage Limit is reduced when you use all your processors (as selected in the Host Processing pop-up menu). For example, on dual-processors, the limit is 90%. On four-processor computers, the limit is 95%.
*
Increasing the CPU Usage Limit may slow down screen responses on slower computers.
To change the CPU Usage Limit:
1 Choose Setup > Playback Engine.
2 From the CPU Usage Limit pop-up menu, select the percentage of CPU processing you want to allocate to Pro Tools.
3 Click OK.