I play a number of games on my PC and I find that whenever I enable to FSAA setting, 2X 4X or 8X the game become really laggy compared to what it would be when the setting is off. Why does this setting cause the GPU to be used so much more?
Cause the way this setting works is very demanding. It applies Antialiasing in entire image (FSAA - Full Scene Anti Aliasing).
So your VGA needs to calculate how to redraw every single pixel. Each frame.
See.. if your game runs at 1080p@60fps this makes roughly 124416000 pixels draws each second. If you turn FSAA on. Your VGA will need to draw and recalculate each of this 124M pixels per second.