Extremely poor performance regardless of settings

In a nutshell, the game runs at 3 frames per second. It doesn't matter if it's low settings, high settings, full screen, screen resolution, nothing changes. I've attempted every answer I could find. Restarted my computer, ran as admin, uninstalled and reinstalled, verified game cache, updated drivers, changed priority in Task Manager, you name it. Nothing works.

After typing that short rant, it occurred to me that I should let you know what my rig is. GTX 960, 24 GB RAM, i5 4460 3.2 GHz.

A couple other details include the game's performance being silky smooth for about two seconds when I open the Steam overlay, and I also can't visually tell the difference between low and high graphics quality.