Maybe your i3 is holding back your PC in this game.
yes im already put the gtx 760 on 16x pci (the longest blue one) and already put all power pin connector , even if my i3 holding back / bottleneck will not be really crazy like this (seriously ? 1280 x 720 low setting all water reflect declined still lag as hell 🙁
Its your pc and/or the game itself. To check how your PC is running Download HWInfo and run Sensors. Post the results - usually 3 screenshots in TinyPic or similar.