Blender currently has 2 algorithms for merging vertices:
- `BKE_mesh_merge_verts`;
- `blender::geometry::create_merged_mesh`
`BKE_mesh_merge_verts` has a simplified algorithm to work with Array,
Mirror and Screw modifiers. It doesn't support merge results that would
create new faces. However it has shortcuts to be more efficient in
these modifiers.
`blender::geometry::create_merged_mesh` tries to predict all possible
outcomes. So it's a more complex. But it loses in performance to
`BKE_mesh_merge_verts` in some cases.
The performance comparison between these two depends on many factors.
`blender::geometry::create_merged_mesh` works with a context that has
only the affected geometry. Thus a smaller region of the mesh is read
for duplicate checking. Therefore, the smaller the affected geometry,
the more efficient the operation.
By my tests `blender::geometry::create_merged_mesh` beats
`BKE_mesh_merge_verts` when less than 20% of the geometry is affected
in worst case `MESH_MERGE_VERTS_DUMP_IF_EQUAL` or 17% in case of
`MESH_MERGE_VERTS_DUMP_IF_MAPPED` .
For cases where the entire geometry is affected, a 30% loss was noticed,
largely due to the creation of a context that represents the entire mesh.
Germano Cavalcante