Low grass/tree performances SRP

I am trying to add grass to my proceduraly generated terrain and I don't understand why my grass is killing the performances. The grass clumps are one prefab added as tree to match with Unity terrrain system with LODs. My shader is SRP compatible and SRP is enable. GPU instanciated is enable on the Material. Occlusion culling too. My terrain is Huge so there is a bunch of clumps in the scene but there are not so much in the camera view.
I wanted to get some grass field at first but it is running at 10fps with only dozen of low poly prefabs...