I can’t tell you anything very exciting I’m afraid.
Foliage is always a total juggling act. Obvious areas to look at are:
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LODs and textures of the meshes.
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Cull distance
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Shadows of the directional light ( I’m assuming it’s moveable )
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Density of the foliage
One thing I would say is, try it with some simple grass assets, there are plenty of free ones on the market. Quixel assets are no compromise in all respects, probably has a lot to do with it.
Also, if you ever get a copy of any well written landscape / foliage solution from the market, you’ll notice that about 60 fps is considered acceptable. You’ll be hard pushed to get above that with any decent amount of coverage.