System Configuration : System Setup : System Usage : DSP Caching

DSP Caching
(Pro Tools|HD Systems Only)
Pro Tools|HD systems maintain DSP caching (Plug-In and Mixer caching for allocated DSP) when closing and opening sessions. While this caching has no effect on the time it takes to open the first session after you launch Pro Tools, it does result in being able to open and close all subsequent Pro Tools sessions much more quickly, especially when using the Revert To Saved command or when opening similarly configured sessions.
However, when using DSP caching, the System Usage window may not always accurately show the DSP resources your session is currently using. For an accurate display of current DSP usage, you can either disable DSP caching or purge the DSP cache.
To enable (or disable) DSP Caching:
1 Open the System Usage window (Windows > System Usage).
2 Click the DSP Cache pop-up menu and select (or deselect) Use DSP Cache.
 
To purge the DSP cache:
1 Open the System Usage window (Windows > System Usage).
2 Click the DSP Cache pop-up menu and select (or deselect) Purge Cache. This refreshes the System Usage display.