# UNDF: UNDF-2026-000001165
--- a/synfig-studio/src/synfigapp/actions/layerduplicate.cpp
+++ b/synfig-studio/src/synfigapp/actions/layerduplicate.cpp
@@ -40,6 +40,7 @@
 #include <synfigapp/canvasinterface.h>
 #include <synfigapp/localization.h>

+#include <unordered_set>
 #endif

@@ -112,19 +113,22 @@ get_top_parent_if_inline_canvas(Canvas::Handle canvas)
 /// Remove the layers that are inside an already listed group-kind layer, as they would be duplicated twice
 static std::list<Layer::Handle>
 remove_layers_inside_included_pastelayers(const std::list<Layer::Handle>& layer_list)
 {
-	std::vector<Layer::Handle> layerpastecanvas_list;
+	// Build a hash set of selected paste-canvas layers for O(1) membership test.
+	// The original std::vector + std::find was O(P) per ancestor walk step,
+	// producing O(L * D * P) total where L = layers, D = nesting depth,
+	// P = paste-canvas count.  With a hash set the inner find drops to O(1)
+	// and the overall complexity becomes O(L * D).
+	std::unordered_set<Layer*> layerpastecanvas_set;
 	for (const auto& layer : layer_list) {
 		if (dynamic_cast<Layer_PasteCanvas*>(layer.get())) {
-			layerpastecanvas_list.push_back(layer);
+			layerpastecanvas_set.insert(layer.get());
 		}
 	}

 	std::list<Layer::Handle> clean_layer_list;
 	for (const Layer::Handle& layer : layer_list) {
 		bool is_inside_a_selected_pastecanvas = false;
 		auto parent_paste_canvas = layer->get_parent_paste_canvas_layer();
 		while (parent_paste_canvas) {
-			if (std::find(layerpastecanvas_list.begin(), layerpastecanvas_list.end(), parent_paste_canvas) != layerpastecanvas_list.end()) {
+			if (layerpastecanvas_set.count(parent_paste_canvas.get())) {
 				is_inside_a_selected_pastecanvas = true;
 				break;
 			}