Game running incredibly slow from launch?

[Olipie]

Have a bit of a weird one. Installed the game recently to play with a friend, ran perfectly fine on highest settings with 120fps in our first session. Week later, when I launch it I get 5fps in the menus and in game. This slowdown literally came from nowhere, and changing the graphics settings has no effect. Have tried lowest settings, highest and even windowing the game. Any help would be much appreciated since it is unplayable like this.