I think the reason why performance has taken such a huge hit is because they are:
1. Not using mipmaps. The idea behind mipmaps is that the farther something is away, the smaller their texture resolution size should be as you'll not notice most of the details anyway. So, to illustrate with an example:
-> If you used mipmaps, then up close it would be say 1k texture (1024x1024). Then when you walk say 10 meters away, it now uses the 0.5k texture (512x512), then at 20 meters away the 0.25k texture (256x256). I don't think they are using mipmaps, so at 20 meters away, everything will still be a 1k texture.
2. Not using draw distance controls. If you don't use them, then it will always try to draw / render things at maximum distance.
3. Not using Occulsion Culling. This is a render technique that only renders things that a player can see. If you aren't using it, it renders everything conceivably in your viewport, based on the direction you are facing and the draw distance from the player. Example:
-> If there is a hill, with plants on the other side of the hill, with Occulsion Culling on, the game will not render those plants until you can see them. If it is off, it will render them, even if you can't see them.
4. Their post-processing effects are poorly designed. With UE4, you want fewer particle generators, as these are super expensive, computationally... can have them produce as many particles as you want, but it is the number of generators that kills framerate. Instead of having one particle generator for the entire forest to generate the fog, there is a generator every couple meters.
5. The size of the game hasn't increased considerably. So I'm thinking they are procedurally generated the plants at runtime, rather than having them fixed ingame, built into the maps. This causes a massive performance hit.
Those are the likely culprits...