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Hello, the best way to optimize meshes on a big landscape in unreal engine 5.1 and beyond is to set Lods on the mesh
First go into the mesh editor set the Lod group to foliage. Then set number of Lods too 8 click the + and there might be and option that comes up click yes. Next find where it says screen size and choose one of the Lods from the drop-down window. Then go find one of the Lods you like and set the other Lods that came behind it to 99 for the screen size so it will never activate. Then for the Lod you like go and zoom close to the mesh and look at the screen size and then put that screen size into the Lod group and click + Continue the steps and slowly zoom out
Down below is what mine looks like with a mesh
