diff --git a/src/Build/BackEnd/Components/RequestBuilder/BatchingEngine.cs b/src/Build/BackEnd/Components/RequestBuilder/BatchingEngine.cs
index 23d9c14397a..2b79814029e 100644
--- a/src/Build/BackEnd/Components/RequestBuilder/BatchingEngine.cs
+++ b/src/Build/BackEnd/Components/RequestBuilder/BatchingEngine.cs
@@ -360,6 +360,8 @@ private static List<ItemBucket> BucketConsumedItems(
                             buckets.Insert(~matchingBucketIndex, matchingBucket);
                         }
 
+                        matchingBucket.Lookup.EnsureCapacity(items.Count);
+
                         // We already have a bucket for this type of item, so add this item to
                         // the bucket.
                         matchingBucket.AddItem(item);
diff --git a/src/Build/BackEnd/Components/RequestBuilder/Lookup.cs b/src/Build/BackEnd/Components/RequestBuilder/Lookup.cs
index 8a275483719..7448add2d57 100644
--- a/src/Build/BackEnd/Components/RequestBuilder/Lookup.cs
+++ b/src/Build/BackEnd/Components/RequestBuilder/Lookup.cs
@@ -376,7 +376,7 @@ private void MergeScopeIntoLastScope()
             // adds to the world
             if (PrimaryAddTable != null)
             {
-                SecondaryTable ??= new ItemDictionary<ProjectItemInstance>();
+                SecondaryTable ??= new ItemDictionary<ProjectItemInstance>(PrimaryAddTable.ItemTypesCount);
                 SecondaryTable.ImportItems(PrimaryAddTable);
             }
 
@@ -706,6 +706,24 @@ internal void AddNewItemsOfItemType(string itemType, ICollection<ProjectItemInst
             PrimaryAddTable.ImportItemsOfType(itemType, itemsToAdd);
         }
 
+        /// <summary>
+        /// Attempts to set the initial capacity of the primary add table.
+        /// </summary>
+        /// <param name="capacity"></param>
+        internal void EnsureCapacity(int capacity)
+        {
+            if (PrimaryAddTable is null)
+            {
+                PrimaryAddTable = new ItemDictionary<ProjectItemInstance>(capacity);
+            }
+#if NET
+            else
+            {
+                PrimaryAddTable.EnsureCapacity(capacity);
+            }
+#endif
+        }
+
         /// <summary>
         /// Implements a true add, an item that has been created in a batch.
         /// </summary>
@@ -812,7 +830,7 @@ internal void ModifyItems(string itemType, ICollection<ProjectItemInstance> grou
             }
         }
 
-        #endregion
+#endregion
 
         #region Private Methods
 
diff --git a/src/Build/BackEnd/TaskExecutionHost/TaskExecutionHost.cs b/src/Build/BackEnd/TaskExecutionHost/TaskExecutionHost.cs
index a8ea306fb91..03b4ee3d7bd 100644
--- a/src/Build/BackEnd/TaskExecutionHost/TaskExecutionHost.cs
+++ b/src/Build/BackEnd/TaskExecutionHost/TaskExecutionHost.cs
@@ -1384,6 +1384,7 @@ private void GatherTaskItemOutputs(bool outputTargetIsItem, string outputTargetN
                 {
                     // Only count non-null elements. We sometimes have a single-element array where the element is null
                     bool hasElements = false;
+                    _batchBucket.Lookup.EnsureCapacity(outputs.Length);
 
                     foreach (ITaskItem output in outputs)
                     {
@@ -1548,6 +1549,8 @@ private void GatherArrayStringAndValueOutputs(bool outputTargetIsItem, string ou
             {
                 if (outputTargetIsItem)
                 {
+                    _batchBucket.Lookup.EnsureCapacity(outputs.Length);
+
                     // to store the outputs as items, use the string representations of the outputs as item-specs
                     foreach (string output in outputs)
                     {
diff --git a/src/Build/Collections/ItemDictionary.cs b/src/Build/Collections/ItemDictionary.cs
index 39834b4dafa..fbe1ca4cd82 100644
--- a/src/Build/Collections/ItemDictionary.cs
+++ b/src/Build/Collections/ItemDictionary.cs
@@ -82,7 +82,27 @@ public ItemDictionary(IEnumerable<T> items)
         /// <summary>
         /// Number of items in total, for debugging purposes.
         /// </summary>
-        public int Count => _nodes.Count;
+        public int Count
+        {
+            get
+            {
+                lock (_itemLists)
+                {
+                    return _nodes.Count;
+                }
+            }
+        }
+
+        public int ItemTypesCount
+        {
+            get
+            {
+                lock (_itemLists)
+                {
+                    return _itemLists.Count;
+                }
+            }
+        }
 
         /// <summary>
         /// Get the item types that have at least one item in this collection
@@ -128,6 +148,16 @@ public ICollection<T> this[string itemtype]
             }
         }
 
+#if NET
+        public void EnsureCapacity(int desiredCapacity)
+        {
+            lock (_itemLists)
+            {
+                _itemLists.EnsureCapacity(desiredCapacity);
+            }
+        }
+#endif
+
         /// <summary>
         /// Empty the collection
         /// </summary>