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This is a paid asset, but now you can download Multiple Terrain Optimizer Free.
Multiple Terrain Optimizer v1.2
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Have you got lots of terrain in your scene? Optimize it! This is a simple and easy to use script that enables/disables terrain based off distance and camera angle.
Just put the camera into script on transform place and put your terrain to Terrain tag. Adjust distance on that you would like to cut terrains – if you need it. You don’t lose collisions or any other information about terrain during this process.