Low clock CPU coupled with extreme graphics settings while running in windowed mode, well you could not have made it worse, no wait, you did, install it on an HDD.
With Intel you need above 4 GHz and a 6400 is kind of a potato of a CPU.
GPU wise you can run above 60 fps even with an R9 280x, it's all about settings and being sensible.
Not using an SSD for game installs is universally stupid.
GTA is GTA and riddle with issues, so even if you run in the ideal 100 fps range you can still have problems due to the game. If you run around or above 150 fps the game breaks entirely.