diff --git a/ICSharpCode.Decompiler/CSharp/Syntax/AstNode.cs b/ICSharpCode.Decompiler/CSharp/Syntax/AstNode.cs
index 4a98052c5..8f61e62e5 100644
--- a/ICSharpCode.Decompiler/CSharp/Syntax/AstNode.cs
+++ b/ICSharpCode.Decompiler/CSharp/Syntax/AstNode.cs
@@ -539,10 +539,20 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 						throw new ArgumentException("Node is already used in another tree.", nameof(value));
 				}
 			}
-			field?.ClearParentAndIndex();
+			T? oldField = field;
+			int oldChildIndex = oldField?.childIndex ?? -1;
+			oldField?.ClearParentAndIndex();
 			field = newValue;
 			newValue?.SetParent(this);
-			InvalidateChildIndices();
+			// A single slot always occupies the same flattened index, so replacing its child in place
+			// changes no index: carry the slot's index to the new child instead of invalidating (and
+			// rebuilding) every child's index. Setting or clearing the slot still invalidates, since the
+			// new child's index is then not known here. When the indices are already stale the carried
+			// value is corrected by the next EnsureChildIndices anyway.
+			if (oldField != null && newValue != null)
+				newValue.childIndex = oldChildIndex;
+			else
+				InvalidateChildIndices();
 		}
 
 		internal void SetParent(AstNode newParent)
diff --git a/ICSharpCode.Decompiler/CSharp/Syntax/AstNodeCollection.cs b/ICSharpCode.Decompiler/CSharp/Syntax/AstNodeCollection.cs
index 3b1ce699b..6ba98154b 100644
--- a/ICSharpCode.Decompiler/CSharp/Syntax/AstNodeCollection.cs
+++ b/ICSharpCode.Decompiler/CSharp/Syntax/AstNodeCollection.cs
@@ -98,10 +98,14 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 				if (old == value)
 					return;
 				ValidateNewChild(value);
+				int oldChildIndex = old.childIndex;
 				old.ClearParentAndIndex();
 				list[index] = value;
 				value.SetParent(parent);
-				parent.InvalidateChildIndices();
+				// Replacing an element in place keeps its flattened position, so no index changes: carry
+				// the old element's index to the new one rather than invalidating (and rebuilding) the
+				// parent's whole index set. A stale carried value is corrected by the next renumber.
+				value.childIndex = oldChildIndex;
 			}
 		}