Oculus rift DK2 - How to increase FPS/refresh rate?

[TNCreator]

Hi,
I tried to use my dk2 with Subnautica, but the framerate is not high enough to make it enjoyable.
I play with minimum settings with an HD7970.
When playing on my normal screen, it runs super nice with experimental graphics at 1920*1200

Is there any tweak to make the game run at 75 fps? is it a refresh setting problem?

others demos & games like Elite:Dangerous run really nice at 75fps on this pc.
Subnautica could be awesome with the Rift but with a low framerate it just got me sick :(

thanks in advance.