Are the lights on? I was having the exact same issue and couldn't figure it out for almost a week. Last night I was playing and realized the lights were on during the day still and long behold as soon as i turned them off my game ran perfect.
When the shed lighting is off, the FPS in the game is normal, but when the shed lighting is on, and you are in range and even look in the direction of the lighting, the FPS tanks... it goes from a smooth and stable 60 FPS to like 17 FPS.
