The Composite extension for X causes an entire sub-tree of the window hierarchy to be rendered to an off-screen buffer. Applications can then take the contents of that buffer and do whatever they like. The off-screen buffer can be automatically merged into the parent window or merged by external programs, called compositing managers.
The Composite extension can't be used simultaneously with Xinerama. However, it can be used simultaneously with Nvidia Twinview.
Disabling the extension
The composite extension is enabled by default. To disable it, add this to xorg.conf or a file in /etc/xorg.conf.d:
Section "Extensions" Option "Composite" "Disable" EndSection
This may slightly improve your performance with some drivers.