What you should have done was stress test, bench, the system as new, say using futuremark and Furmark. Now you could reinstall and bench and see if the degrade was permanent.
Most likely it's a software problem. But you need to erase the drive, either SSD or HDD, not format HDD but erase, or secure erase the SSD, reinstall windows, then bench the CPU and look at other benches for the same CPU on a close system and the same for the GPU.
This will tell you if there was a drop in the system performance. The GPU is what gives you performance, if you have a GTX 460 vs a GTX 680 which one has a better FPS, the CPU does play a role but more then likely the GPU is the first thing that could be the problem under software, such as the game stores and windows.
Furmark, you can look at others that have benched with the same CPU, GPU, and get a idea, because you didn't when the system was new so you don't have a comparison of your own system.
Either way, if you clean install, and run the game and get 60fps again, it was most likely a software problem.
If you run a PSU calc for your system and at around the min for the PSU, your stressing it to much, go with at least 200 above the min.