# UNDF: UNDF-2026-000001156
--- a/core/src/display_object/movie_clip.rs
+++ b/core/src/display_object/movie_clip.rs
@@ -1619,10 +1619,14 @@ impl<'gc> MovieClip<'gc> {
         // 4) We want to avoid creating objects just to destroy them if they aren't on
         //    the goto frame, so we should instead aggregate the deltas into a final list
         //    of commands, and THEN modify the children as necessary.
-
-        // This map will maintain a map of depth -> placement commands.
-        // TODO: Move this to UpdateContext to avoid allocations.
-        let mut goto_commands: Vec<GotoPlaceObject<'_>> = vec![];
+
+        // This map will maintain a map of depth -> placement commands.
+        // TODO: Move this to UpdateContext to avoid allocations.
+        let mut goto_commands: Vec<GotoPlaceObject<'_>> = vec![];
+        // O(1) depth lookup index: depth -> index in goto_commands Vec.
+        // Previously each goto_place_object/goto_remove_object call did
+        // iter().position() O(D) per SWF tag; a gotoAndPlay spanning F frames
+        // with D display objects per frame costs O(F*D^2). HashMap brings it to O(F*D).
+        let mut goto_depth_index: std::collections::HashMap<swf::Depth, usize> =
+            std::collections::HashMap::new();

@@ -1688,8 +1692,12 @@ impl<'gc> MovieClip<'gc> {
                     Action::Place(version) => {
                         index += 1;
                         self.0.goto_place_object(
+                            &mut goto_depth_index,
                             reader,
                             version,
                             &mut goto_commands,
                             is_rewind,
                             index,
                         )
                     }
                     Action::Remove(version) => self.goto_remove_object(
+                        &mut goto_depth_index,
                         reader,
                         version,
                         context,
@@ -2143,6 +2151,7 @@ impl<'gc> MovieClip<'gc> {
     fn goto_remove_object<'a>(
         mut self,
         reader: &mut SwfStream<'a>,
         version: u8,
         context: &mut UpdateContext<'gc>,
+        goto_depth_index: &mut std::collections::HashMap<swf::Depth, usize>,
         goto_commands: &mut Vec<GotoPlaceObject<'a>>,
         is_rewind: bool,
         from_frame: FrameNumber,
@@ -2158,8 +2167,14 @@ impl<'gc> MovieClip<'gc> {
         }?;
         let depth: Depth = remove_object.depth.into();
-        if let Some(i) = goto_commands.iter().position(|o| o.depth() == depth) {
-            goto_commands.swap_remove(i);
+        if let Some(&i) = goto_depth_index.get(&depth) {
+            let last_depth = goto_commands.last().map(|o| o.depth());
+            goto_commands.swap_remove(i);
+            goto_depth_index.remove(&depth);
+            // swap_remove moves the last element to position i; update its index entry.
+            if let Some(d) = last_depth {
+                if d != depth {
+                    goto_depth_index.insert(d, i);
+                }
+            }
         }

@@ -3301,6 +3316,7 @@ impl<'gc> MovieClip<'gc> {
     fn goto_place_object<'a>(
         &self,
         reader: &mut SwfStream<'a>,
         version: u8,
+        goto_depth_index: &mut std::collections::HashMap<swf::Depth, usize>,
         goto_commands: &mut Vec<GotoPlaceObject<'a>>,
         is_rewind: bool,
         index: usize,
@@ -3325,10 +3341,15 @@ impl<'gc> MovieClip<'gc> {
         let depth: Depth = place_object.depth.into();
         let mut goto_place = GotoPlaceObject::new(
             self.current_frame(),
             place_object,
             is_rewind,
             index,
             tag_start,
             version,
         );
-        if let Some(i) = goto_commands.iter().position(|o| o.depth() == depth) {
-            goto_commands[i].merge(&mut goto_place);
+        if let Some(&i) = goto_depth_index.get(&depth) {
+            goto_commands[i].merge(&mut goto_place);
         } else {
+            goto_depth_index.insert(depth, goto_commands.len());
             goto_commands.push(goto_place);
         }