EDIT: First, make sure your Nvidia drivers are the newest available (Stable).
To help see which piece of hardware is slowing you down, please do the following:
Open task manager and click on open resource monitor (Located on the lower left). Then click on the cpu tab. Run the game. After playing for a while, alt-tab to task manager.
Quickly: (so that the recorded CPU data is still from the game and not from outside of the game)
See what % utilization the CPU is. Also, note the speed. If your CPU is thermal throttling, the speed will be lower than 3.5 GHZ.
Then, go to resource manager and scroll down the right side. See if any of the cores (Called CPU#) maxed out.
This will let us know if your CPU is the problem.
The other thing to do is:
Use MSI afterburner or something similar. Do a similar process as above, but with GPU utilization and clocks / temps. To see if the GPUs are maxed out or thermal throttling.