AMD gpus have a serious problem with performance for whatever reasons when you reach the sea location. And it gets even worse at the Beast location and after that until the very end (except for the final fight area). Regardless of settings used.(low or high, it doesn't matter)
I'm talking about 25-27+ fps @1920x1200 on my firstname.lastname@example.org 16GB amd fury 1100\500 unlocked.
Steam forums are full of angry AMD players with different hardware who reached these areas. I seriously hope either AMD or the devs will address this issue.
PS http://i.imgur.com/mv4pxLJ.jpg And that's not the worst fps I've seen.At the Beast location fps often dropped to 25- fps or even lower