You can combine two or more meshes into one polygon object using the Combine Meshes feature. The Combine Meshes command assembles the meshes as polygon shells in the new polygon mesh node that gets created. The topology of the combined meshes is not modified in any other way.
In general, merging meshes together object increases performance. The more unique objects in your scene, the more time is needed to compute the mesh.
To combine two or more polygon meshes into one mesh