I hope this helps.
I took a lot of screen shots with different graphic settings. The settings should be quite visible. I tested it pre-screenshot and got what I felt was an analogous response with higher settings, which I’m not sure if it skews or improperly correlates with your results. By that I mean, with higher settings I think I got the same results as you.
I’m sorry there is so many screenshots, I was trying to get as close to my pre-screenshot setting without the benefit of knowing what it was, so I just tried several settings. My last two, should ultimately demonstrate a dip to a lower FPS, granted with the benefit of high rendering setting. My first few screenshots should be close to your settings. There is still a dip, but just not the kind of dip that I think you wanted to see.
My computer hardware is:
GeForce GTX 680 2GB 256bit GDDR5
Intel Core i73700K
16GB of RAM
Seagate Barracudea 2TB 7200 RPM 64MB Cache
Two Monitor Setup
(edited by rentalpep.6427)