First, this happened.
https://secure.gamesas.com/forums/viewtopic.php?f=40&t=39506

Nobody responded, but that's okay , system spec included in that link. (if you don't read the link you won't understand the rest)

Flow of events:
1) Game released. I ran the game on very high settings all the way though.
2) 1.9 + DX11 and high res packs came out, installed them both, while keeping settings on very high. I was also using an HDMI cable, which caused the popular brackets around my full screened game, so I began the habit of booting the game in windowed mode, and putting into full screen as soon as it hit the menu, to avoid having the brackets.
3) I got tired of HDMI being annoying, started using DVI. **** important****
4) I can now boot the game in full screen, and never have to mess with that option again, except now, I'm dropping down to 30fps in frequent spots, in the past I never dropped below 50. Ever. Anywhere. Then I discovered if I ***put the game into windowed mode, it ran perfectly like it used too, I could even put it in full screen again, and it would be perfect. I did this in-game, so it took me a while to make the connection...
5) Having completely forgotten that I practically did the same thing with HDMI, I gave it a test. While still using DVI, i booted the game in windowed mode, and switched it to full screen, in the MENU. Game runs like butter. So no matter what, I cannot boot this game in full screen, or I get a major loss of FPS.

Lastly, I'm glad I figured this out. But I don't know if something get's disabled if I start the game in windowed mode, or if this is just a really, really, bad bug. But, if someone else had issues with this, I hope I helped.

And what's wrong with 30 FPS ?

Well, after saving up money, perfectly aligning deals + quality parts, a lot is wrong is 30fps. I've played every current game at solid 55-60 with vsync on, and some with it off, at around 120+. And, after having so many frames for so long, 30 is really, REALLY noticeable...

If I was poor, or was given a free computer (beg, not chosing) there would be nothing wrong with it. But this isn't a rig built for Minecraft

What do you use to measure your fps?

Well i read around and tried to see whether or not the console command worked for Crysis 2, and after a while I just gave into fraps (which also displays on my Logitech keyboard ).

So Fraps.

If I start the game in full screen then alt-enter to windowed mode, and alt-enter back into fullscreen, my vram usage drops from 744/mb (maxed on my sli 768gb 460s) to about 350gb-450gb and frame rates / stuttering improve dramatically. The vram usage will slowly climb until I get stutter and have to "reset" it again.

Your situation sounds similar?