Here is a list of things to try:
First: Run Task Manager while playing
1. CPU is 100%: This is due to the resolution and/or settings being TOO LOW. Raise them up so your GPU can take them from the CPU.
2. GPU is 100%: This is due to the resolution and/or settings being TOO HIGH. Lower them a little so your GPU can keep up.
I can almost guarantee the issue is due to the settings/resolution being too high. The MAXIMUM FPS is determined by the CPU. In other words, if you
lower the settings to the bare minimum, it will show you how fast you can get the game to run. For example, my i7-3770K can run the game at about 140 FPS at max with minimum settings. Once you start turning the settings up, while this does make the game look nicer, and transfers a lot of the load to the GPU, you need to remember that the CPU is FASTER than the GPU, and the higher the resolution is, the slower the GPU will go, thus, decreasing the maximum output of FPS. Start with the settings, for example:
at 1440p or 2160p it is almost completely unnecessary to use the FXAA settings or any other AA settings.
On a side note, make sure you aren't using super sampling in the NVidia control panel. This will force the game to a higher resolution while lowering performance. It's best to use the option: "let the 3D application decide"