Thanks for your response! I didn't mean to imply Sunken had optimization issues, I just meant that it was something common I come across that can make my GPU lag.
I discovered something baffling. After trying a bunch of options in the nVidia settings, I gave up on finding anything that worked and just restored it to default settings so I could start over.
And as it turns out, the game actually runs better on my integrated Intel graphics chip than the dedicated nVidia card. I still have a few frame drops due to draw distance, but otherwise it runs fine. Completely the opposite of what it should be, but at least once I can turn off all the lights I'll be able to play it. Yay!