I found this option in CCSM. It's unchecked by default. There's a tooltip explaining that it allows drawing on fullscreen windows to not be redirected to offscreen pixmaps.
This option causes drawing to occur directly on the screen rather than drawing off-screen and then copying the drawn content back onto the screen.
Thus, yes it can dramatically improve graphical performance.
However there can be a draw-back - on some graphics cards/monitors - using this option can result in dramatic flickering. Particularly - this flickering effect can occur on full-screen apps.
Thus - its work experimenting, but if you see flickering, its best not to use.
Remember using CCSM can break your desktop.
If you want to toggle this value, you can use: