System Configuration : System Setup : Configuring Pro Tools System Settings : Optimizing the Plug-In Streaming Buffer Size

Optimizing the Plug-In Streaming Buffer Size
(Structure Plug-In Only)
This option appears in the Playback Engine only if the Structure plug-in is installed on your system. This option is useful when you are playing samples from the same drive that contains audio for the current session. When this option is selected, Pro Tools automatically optimizes the size of the Plug-In Streaming Buffer to facilitate disk access from both Pro Tools and Structure. The Plug-In Streaming Buffer Size pop-up menu is unavailable when this option is selected.
To set Pro Tools to automatically optimize the Plug-In Streaming Buffer Size:
1 Choose Setup > Playback Engine.
2 Select the Optimize for Streaming Content on Audio Drives option.
3 Click OK.