Mesh Merge and Splice provides a simple way to help reduce draw calls within Unity as multiple objects into one. It combines meshes and sub-meshes into one more larger and more efficient mesh.
Rendering 3D objects is a complicated process. For each mesh, Unity needs to package up the textures, gather the mesh data, send everything to the graphics API, set the render state, and finally render the objects. If your scene contains a lot of objects, you’ll see your draw calls increase and performance decrease.
Unity will do tricks to help reduce the draw calls including batching objects, but one of the best things you can do is reduce the number of objects that need to be drawn. While it may not be obvious, it’s much better to render a small number of large objects than it is to render a large number of small objects.
In the image below, we could render out 150 individual mushrooms (with 200 vertices each) or we could render 1 mushroom group with 30,000 vertices (150 x 200). The latter is much faster.