Performance improvements:

Lazy "Place on face" gizmo update.
Caching of MeshObject::raw_mesh_bounding_box() for the object size display.
ModelObject::bounding_box(), raw_mesh_bounding_box(), full_raw_mesh_bounding_box() will not copy the mesh.
TriangleMesh::transformed_bounding_box(const Transform3d &trafo) will not copy the mesh data.
get_options_for_bundle() will not return reference to temp value
is_splittable() calls cheap mesh.has_multiple_patches()

src/slic3r/GUI/GLGizmo.cpp

@@ -1428,6 +1428,7 @@ void GLGizmoFlatten::on_start_dragging(const GLCanvas3D::Selection& selection)
 {
     if (m_hover_id != -1)
     {
+        assert(m_planes_valid);
         m_normal = m_planes[m_hover_id].normal;
         m_starting_center = selection.get_bounding_box().center();
     }
@@ -1446,6 +1447,8 @@ void GLGizmoFlatten::on_render(const GLCanvas3D::Selection& selection) const
         ::glPushMatrix();
         ::glMultMatrixd(m.data());
         ::glTranslatef(0.f, 0.f, selection.get_volume(*selection.get_volume_idxs().begin())->get_sla_shift_z());
+        if (this->is_plane_update_necessary())
+			const_cast<GLGizmoFlatten*>(this)->update_planes();
         for (int i = 0; i < (int)m_planes.size(); ++i)
         {
             if (i == m_hover_id)
@@ -1478,6 +1481,8 @@ void GLGizmoFlatten::on_render_for_picking(const GLCanvas3D::Selection& selectio
         ::glPushMatrix();
         ::glMultMatrixd(m.data());
         ::glTranslatef(0.f, 0.f, selection.get_volume(*selection.get_volume_idxs().begin())->get_sla_shift_z());
+        if (this->is_plane_update_necessary())
+			const_cast<GLGizmoFlatten*>(this)->update_planes();
         for (int i = 0; i < (int)m_planes.size(); ++i)
         {
             ::glColor3f(1.0f, 1.0f, picking_color_component(i));
@@ -1497,11 +1502,11 @@ void GLGizmoFlatten::on_render_for_picking(const GLCanvas3D::Selection& selectio
 void GLGizmoFlatten::set_flattening_data(const ModelObject* model_object)
 {
     m_starting_center = Vec3d::Zero();
-    bool object_changed = m_model_object != model_object;
+    if (m_model_object != model_object) {
+        m_planes.clear();
+        m_planes_valid = false;
+    }
     m_model_object = model_object;
-
-    if (model_object && (object_changed || is_plane_update_necessary()))
-        update_planes();
 }
 
 void GLGizmoFlatten::update_planes()
@@ -1701,6 +1706,8 @@ void GLGizmoFlatten::update_planes()
     }
     m_first_instance_scale = m_model_object->instances.front()->get_scaling_factor();
     m_first_instance_mirror = m_model_object->instances.front()->get_mirror();
+
+    m_planes_valid = true;
 }
 
 
@@ -1709,7 +1716,7 @@ bool GLGizmoFlatten::is_plane_update_necessary() const
     if (m_state != On || !m_model_object || m_model_object->instances.empty())
         return false;
 
-    if (m_model_object->volumes.size() != m_volumes_matrices.size())
+    if (! m_planes_valid || m_model_object->volumes.size() != m_volumes_matrices.size())
         return true;
 
     // We want to recalculate when the scale changes - some planes could (dis)appear.

src/slic3r/GUI/GLGizmo.hpp

@@ -408,6 +408,7 @@ private:
     Vec3d m_first_instance_mirror;
 
     std::vector<PlaneData> m_planes;
+    bool m_planes_valid = false;
     mutable Vec3d m_starting_center;
     const ModelObject* m_model_object = nullptr;
     std::vector<const Transform3d*> instances_matrices;

src/slic3r/GUI/GUI_ObjectList.cpp

@@ -621,7 +621,8 @@ const std::vector<std::string>& get_options_for_bundle(const wxString& bundle_na
         if (bundle_name == _(it.first))
             return it.second;
     }
-    return std::vector<std::string> {};
+	static std::vector<std::string> empty;
+	return empty;
 }
 
 //				  category ->		vector 			 ( option	;  label )
@@ -1276,15 +1277,12 @@ bool ObjectList::is_splittable()
     if (!get_volume_by_item(item, volume) || !volume)
         return false;
 
-    if (volume->is_splittable() != -1) // if is_splittable value is already known
-        return volume->is_splittable() == 0 ? false : true;
-
-    TriangleMeshPtrs meshptrs = volume->mesh.split();
-    bool splittable = meshptrs.size() > 1;
-    for (TriangleMesh* m : meshptrs) { delete m; }
-
-    volume->set_splittable(splittable ? 1 : 0);
-    return splittable;
+	int splittable = volume->is_splittable();
+	if (splittable == -1) {
+		splittable = (int)volume->mesh.has_multiple_patches();
+		volume->set_splittable(splittable);
+	}
+    return splittable != 0;
 }
 
 bool ObjectList::selected_instances_of_same_object()

src/slic3r/GUI/GUI_ObjectManipulation.cpp

@@ -267,7 +267,7 @@ void ObjectManipulation::update_settings_value(const GLCanvas3D::Selection& sele
             bool changed_box = false;
             if (!m_cache.instance.matches_object(obj_idx))
             {
-                m_cache.instance.set(obj_idx, instance_idx, (*wxGetApp().model_objects())[obj_idx]->raw_mesh().bounding_box().size());
+                m_cache.instance.set(obj_idx, instance_idx, (*wxGetApp().model_objects())[obj_idx]->raw_mesh_bounding_box().size());
                 changed_box = true;
             }
             if (changed_box || !m_cache.instance.matches_instance(instance_idx) || !m_cache.scale.isApprox(100.0 * m_new_scale))
@@ -278,7 +278,7 @@ void ObjectManipulation::update_settings_value(const GLCanvas3D::Selection& sele
             m_new_size = Vec3d::Zero();
 #else
         if ((0 <= obj_idx) && (obj_idx < (int)wxGetApp().model_objects()->size()))
-            m_new_size = volume->get_instance_transformation().get_matrix(true, true) * (*wxGetApp().model_objects())[obj_idx]->raw_mesh().bounding_box().size();
+            m_new_size = volume->get_instance_transformation().get_matrix(true, true) * (*wxGetApp().model_objects())[obj_idx]->raw_mesh_bounding_box().size();
         else
             // this should never happen
             m_new_size = Vec3d::Zero();

src/slic3r/GUI/Plater.cpp

@@ -2105,7 +2105,7 @@ void Plater::priv::fix_through_netfabb(const int obj_idx)
         o->clear_instances();
         for (auto instance: model_object->instances)
             o->add_instance(*instance);
-        // o->invalidate_bounding_box();
+        o->invalidate_bounding_box();
         
         if (o->volumes.size() == model_object->volumes.size()) {
             for (int i = 0; i < o->volumes.size(); i++) {