I have the Steam ver. I'm at 13h playtime. FPS dropped to 3 after exploring the "Tree" area.
I suspected it was an OpenGL driver issue, so I updated AMD driver to latest experimental. Still the same, only happening on that one specific save.
Best game I found in years. Hope there is a fix.
*Update: Problem seems to have corrected itself upon leaving the save area, one area ahead, then coming back to the save area. 60 FPS again. The problem replicates when I load the save.