diff --git a/src/UglyToad.PdfPig.DocumentLayoutAnalysis/Clustering.cs b/src/UglyToad.PdfPig.DocumentLayoutAnalysis/Clustering.cs
index bd11c7b5d..6af787324 100644
--- a/src/UglyToad.PdfPig.DocumentLayoutAnalysis/Clustering.cs
+++ b/src/UglyToad.PdfPig.DocumentLayoutAnalysis/Clustering.cs
@@ -216,71 +216,37 @@ public static IEnumerable<IReadOnlyList<T>> NearestNeighbours<T>(IReadOnlyList<T
                 yield return group.Select(i => elements[i]).ToList();
             }
         }
-
-        /// <summary>
-        /// Group elements using Depth-first search.
-        /// <para>https://en.wikipedia.org/wiki/Depth-first_search</para>
-        /// </summary>
-        /// <param name="edges">The graph. edges[i] = j indicates that there is an edge between i and j.</param>
-        /// <returns>A List of HashSets containing the grouped indexes.</returns>
-        internal static List<HashSet<int>> GroupIndexes(int[] edges)
+        
+        internal static List<List<int>> GroupIndexes(int[] edges)
         {
-            int[][] adjacency = new int[edges.Length][];
+            // Improved thanks to https://github.com/UglyToad/PdfPig/issues/1178
+            var adjacency = new List<int>[edges.Length];
             for (int i = 0; i < edges.Length; i++)
             {
-                HashSet<int> matches = new HashSet<int>();
-                if (edges[i] != -1) matches.Add(edges[i]);
-                for (int j = 0; j < edges.Length; j++)
-                {
-                    if (edges[j] == i) matches.Add(j);
-                }
-                adjacency[i] = matches.ToArray();
+                adjacency[i] = new List<int>();
             }
 
-            List<HashSet<int>> groupedIndexes = new List<HashSet<int>>();
-            bool[] isDone = new bool[edges.Length];
-
-            for (int p = 0; p < edges.Length; p++)
-            {
-                if (isDone[p]) continue;
-                groupedIndexes.Add(DfsIterative(p, adjacency, ref isDone));
-            }
-            return groupedIndexes;
-        }
-
-        /// <summary>
-        /// Group elements using Depth-first search.
-        /// <para>https://en.wikipedia.org/wiki/Depth-first_search</para>
-        /// </summary>
-        /// <param name="edges">The graph. edges[i] = [j, k, l, ...] indicates that there is an edge between i and each element j, k, l, ...</param>
-        /// <returns>A List of HashSets containing the grouped indexes.</returns>
-        internal static List<HashSet<int>> GroupIndexes(int[][] edges)
-        {
-            int[][] adjacency = new int[edges.Length][];
+            // one pass O(n) 
             for (int i = 0; i < edges.Length; i++)
             {
-                HashSet<int> matches = new HashSet<int>();
-                for (int j = 0; j < edges[i].Length; j++)
+                int j = edges[i];
+                if (j != -1)
                 {
-                    if (edges[i][j] != -1) matches.Add(edges[i][j]);
+                    // i <-> j
+                    adjacency[i].Add(j);
+                    adjacency[j].Add(i);
                 }
-
-                for (int j = 0; j < edges.Length; j++)
-                {
-                    for (int k = 0; k < edges[j].Length; k++)
-                    {
-                        if (edges[j][k] == i) matches.Add(j);
-                    }
-                }
-                adjacency[i] = matches.ToArray();
             }
 
-            List<HashSet<int>> groupedIndexes = new List<HashSet<int>>();
+            List<List<int>> groupedIndexes = new List<List<int>>();
             bool[] isDone = new bool[edges.Length];
 
             for (int p = 0; p < edges.Length; p++)
             {
-                if (isDone[p]) continue;
+                if (isDone[p])
+                {
+                    continue;
+                }
                 groupedIndexes.Add(DfsIterative(p, adjacency, ref isDone));
             }
             return groupedIndexes;
@@ -290,22 +256,33 @@ internal static List<HashSet<int>> GroupIndexes(int[][] edges)
         /// Depth-first search
         /// <para>https://en.wikipedia.org/wiki/Depth-first_search</para>
         /// </summary>
-        private static HashSet<int> DfsIterative(int s, int[][] adj, ref bool[] isDone)
+        private static List<int> DfsIterative(int s, List<int>[] adj, ref bool[] isDone)
         {
-            HashSet<int> group = new HashSet<int>();
-            Stack<int> S = new Stack<int>();
+            List<int> group = new List<int>();
+            Stack<int> S = new Stack<int>(4);
             S.Push(s);
 
+            isDone[s] = true;
             while (S.Count > 0)
             {
                 var u = S.Pop();
-                if (!isDone[u])
+                group.Add(u);
+
+#if NET
+                var currentAdj = System.Runtime.InteropServices.CollectionsMarshal.AsSpan(adj[u]);
+                int count = currentAdj.Length;
+#else
+                var currentAdj = adj[u];
+                int count = currentAdj.Count;
+#endif
+                for (int i = 0; i < count; ++i)
                 {
-                    group.Add(u);
-                    isDone[u] = true;
-                    foreach (var v in adj[u])
+                    var v = currentAdj[i];
+                    ref bool done = ref isDone[v];
+                    if (!done)
                     {
                         S.Push(v);
+                        done = true;
                     }
                 }
             }