Once a program has finished loading and is inside the memory, the harddisk/ssd will have nothing to do with performance anymore. In theory with a fully loaded game, you can unplug and toss away your disk and nothing should change(don't do that though, the system will likely crash anyway) But as the answer above me stated, if you don't have enough memory and your system has been coming and going back to the secondary storage, then yes the ssd would have helped...