The grass is rendered using a multi mesh instance but runs at only 20 fps its there a better rendering method.
All 3d models were made by me in blender

It's hard to tell your setup just from the screenshot. What exactly are you rendering using multi-mesh instancing? Did you model individual blades of grass? That is way too heavy, even if optimized with multi-mesh instancing. I'm not sure though if that is what you are doing. Please elaborate a bit more on your exact setup.

    award The grass is a texture of individually modelled grass applied using a multimesh to 2m by 3m 3D scans of a patch of bare earth

      Looking into it more, I possibly may have been mistaken about that approach being too heavy. Lots I still need to learn in Godot. Were you perchance following this tutorial?
      It seems to be using the same approach as you.

      The best advice I can give though without knowing more specifics is to profile your scene. See how many tris you're drawing and how many draw calls you're making. See what calls are eating the most of your GPU per frame. You can use the Visual Profiler and Monitors to do these things:

      https://docs.godotengine.org/en/stable/tutorials/scripting/debug/debugger_panel.html