Why after adding grass with LOD causes drop fps?

When I add grass with LOD causes drop fps,what can i do to improve performance?

 

I'd recommend to learn and use the Profiler, it's the only way to gauge where and why performance is being hit. Based on the video alone, it comes down to guessing game.

Adding LODs always triples the amount of Transforms (Root+LOD0+LOD1+etc) in use for that particle asset, which affects CPU load.

 

I mean as the grass becomes invisible(culled) the fps continue to fall
if i add grass as prefab, fps is good.Only i add grass to terrain tools causes drop fps.

 

Check the profiler. You might lod culling taking up a large amount of time for example.