Put GTA V on medium or normal settings at 1080p. Run the game and look at the framerate. Now leaving everything else as is, change the resolution to 720p and run the game again. Did your framerate take a big jump up? If it didn't then this means your CPU is holding you back. Reducing the videocard workload by reducing resolution will cause framerates to go much higher if it's the videocard that can't keep up.