diff --git a/SeeSharp/Integrators/Bidir/BidirBase.cs b/SeeSharp/Integrators/Bidir/BidirBase.cs
index 20a8812..6561688 100644
--- a/SeeSharp/Integrators/Bidir/BidirBase.cs
+++ b/SeeSharp/Integrators/Bidir/BidirBase.cs
@@ -108,6 +108,8 @@ public struct CameraPath {
         /// Roughness of the material at the currently last vertex of the path.
         /// </summary>
         public float CurrentRoughness;
+
+        public int Depth => Vertices.Count;
     }
 
     /// <summary>
diff --git a/SeeSharp/Integrators/Bidir/VertexConnectionAndMerging.cs b/SeeSharp/Integrators/Bidir/VertexConnectionAndMerging.cs
index de9d0d7..05d012a 100644
--- a/SeeSharp/Integrators/Bidir/VertexConnectionAndMerging.cs
+++ b/SeeSharp/Integrators/Bidir/VertexConnectionAndMerging.cs
@@ -121,6 +121,12 @@ protected override void OnAfterRender() {
         Scene.FrameBuffer.MetaData["AverageCameraPathLength"] = AverageCameraPathLength;
         Scene.FrameBuffer.MetaData["AverageLightPathLength"] = AverageLightPathLength;
         Scene.FrameBuffer.MetaData["AveragePhotonsPerQuery"] = AveragePhotonsPerQuery;
+        Scene.FrameBuffer.MetaData["MergeAccelBuildTime"] = mergeBuildTimer.ElapsedMilliseconds;
+    }
+
+    protected override void OnBeforeRender() {
+        base.OnBeforeRender();
+        mergeBuildTimer = new();
     }
 
     protected override void OnStartIteration(uint iteration) {
@@ -137,7 +143,9 @@ protected override void OnCameraPathTerminate(CameraPath path)
 
     private float ComputeAverageLightPathLength() {
         float average = 0;
-        if (LightPaths == null) return 0;
+        if (LightPaths == null)
+            return 0;
+
         for (int i = 0; i < LightPaths.NumPaths; ++i) {
             int length = LightPaths.PathCache.Length(i);
             average = (length + i * average) / (i + 1);
@@ -186,6 +194,8 @@ public override void Render(Scene scene) {
         photonMap = null;
     }
 
+    Stopwatch mergeBuildTimer;
+
     /// <summary>
     /// Generates the acceleration structure for merging
     /// </summary>
@@ -193,6 +203,8 @@ protected override void ProcessPathCache() {
         base.ProcessPathCache();
 
         if (EnableMerging) {
+            mergeBuildTimer.Start();
+
             photonMap.Clear();
             for (int i = 0; i < LightPaths.PathCache.NumVertices; ++i) {
                 var vertex = LightPaths.PathCache[i];
@@ -201,6 +213,8 @@ protected override void ProcessPathCache() {
                 }
             }
             photonMap.Build();
+
+            mergeBuildTimer.Stop();
         }
     }
 
@@ -307,6 +321,7 @@ protected virtual float ComputeLocalMergeRadius(float primaryDistance) {
     protected virtual RgbColor PerformMerging(SurfaceShader shader, RNG rng, CameraPath path, float cameraJacobian) {
         if (path.Vertices.Count == 1 && !MergePrimary) return RgbColor.Black;
         if (!EnableMerging) return RgbColor.Black;
+        if (!MergePrimary && path.Depth == 1) return RgbColor.Black;
         float localRadius = ComputeLocalMergeRadius(path.Distances[0]);
 
         RgbColor estimate = RgbColor.Black;
diff --git a/SeeSharp/Integrators/Common/PathCache.cs b/SeeSharp/Integrators/Common/PathCache.cs
index b430897..1943f13 100644
--- a/SeeSharp/Integrators/Common/PathCache.cs
+++ b/SeeSharp/Integrators/Common/PathCache.cs
@@ -83,6 +83,8 @@ public void Prepare() {
             this.overflow += overflow;
             totalUnused += unused;
         }
+        threadCaches.Values.Clear();
+
         // TODO compact the holes after flushing incomplete caches
     }
 
@@ -124,7 +126,8 @@ class ThreadCache {
             int overflow = next - (vertices.Length - insertPos);
             if (overflow > 0) next -= overflow;
 
-            Array.ConstrainedCopy(batch, 0, vertices, insertPos, next);
+            if (next > 0)
+                Array.ConstrainedCopy(batch, 0, vertices, insertPos, next);
 
             // For uncompleted batches, add a guard
             int unused = overflow > 0 ? 0 : (BatchSize - next);