In case it's not obvious already - The need for 4 buffers in fullscreen mode is due to the simultaneous use of bypass/overlays and framedropping (swap interval 0). Both features require triple buffers, and if used together will require quad-buffers to avoid starving or at least stalling the the pipeline (which is the reduced performance you see).
In case it's not obvious already - The need for 4 buffers in fullscreen mode is due to the simultaneous use of bypass/overlays and framedropping (swap interval 0). Both features require triple buffers, and if used together will require quad-buffers to avoid starving or at least stalling the the pipeline (which is the reduced performance you see).