Could you please try the public beta listed here? https://forums.focus-home.com/topic/5112/public-beta-the-valley-performance-issues/14
This should fix the issue, but let us know if it doesn't.
If you're looking for a way to force 1080p, you can do so with this method: Settings>Display and Sound> Video output > TV Resolution > 1080p
Be aware that this will set your whole console to 1080p, and you will need to revert this if you want to play in 4k again.
I only have a 1080p screen. My One X has always been set for 1080p. Before the update the game seems to have an unlocked frame rate hitting 60fps most of the time and it looked really nice and natural. Now the framerate is downgraded to a locked 30fps.