The game will lock itself to 20FPS and not budge up or down and becomes unplayable. Have re-created the issue on a fresh format and install of Windows 10. I believe it happens when the computer has been put to sleep/hibernation with the game running. CPU & GPU usage will be below 30-40% while gaming is running at 20FPS. Restarts & shutdowns do not solve the issue.
All current drivers.
Load save, exit to main menu, put PC to sleep, wake PC, resume game. Game will be locked at 20fps.