In the GSdx 11 HW hacks, is a performance killer. Most games don't need it. Only enable if you see missing vegetation or transparent objects.
GSdx is a graphics plugin for the PCSX2 emulator, responsible for rendering PS2 graphics on PC. It uses various techniques to translate the PS2's graphics processing unit (GPU) instructions into a format that modern PCs can understand. GSdx is an open-source plugin, which allows developers to modify and improve it continuously. Pcsx2 Gsdx 11 Plugin