diff --git a/src/main/java/graphql/analysis/NodeVisitorWithTypeTracking.java b/src/main/java/graphql/analysis/NodeVisitorWithTypeTracking.java
index 683138e387..2a2b237aeb 100644
--- a/src/main/java/graphql/analysis/NodeVisitorWithTypeTracking.java
+++ b/src/main/java/graphql/analysis/NodeVisitorWithTypeTracking.java
@@ -203,7 +203,7 @@ public TraversalControl visitArgument(Argument argument, TraverserContext<Node>
         QueryVisitorFieldEnvironment fieldEnv = fieldCtx.getEnvironment();
         GraphQLFieldsContainer fieldsContainer = fieldEnv.getFieldsContainer();
 
-        GraphQLFieldDefinition fieldDefinition = Introspection.getFieldDef(schema, fieldsContainer, field.getName());
+        GraphQLFieldDefinition fieldDefinition = Introspection.getFieldDefinition(schema, fieldsContainer, field.getName());
         GraphQLArgument graphQLArgument = fieldDefinition.getArgument(argument.getName());
         String argumentName = graphQLArgument.getName();
 
diff --git a/src/main/java/graphql/execution/ExecutionStrategy.java b/src/main/java/graphql/execution/ExecutionStrategy.java
index 5da3cefbbf..26e126fd79 100644
--- a/src/main/java/graphql/execution/ExecutionStrategy.java
+++ b/src/main/java/graphql/execution/ExecutionStrategy.java
@@ -946,7 +946,7 @@ protected GraphQLFieldDefinition getFieldDef(ExecutionContext executionContext,
      * @return a {@link GraphQLFieldDefinition}
      */
     protected GraphQLFieldDefinition getFieldDef(GraphQLSchema schema, GraphQLObjectType parentType, Field field) {
-        return Introspection.getFieldDef(schema, parentType, field.getName());
+        return Introspection.getFieldDefinition(schema, parentType, field.getName());
     }
 
     /**
diff --git a/src/main/java/graphql/introspection/Introspection.java b/src/main/java/graphql/introspection/Introspection.java
index f20f58d238..ced6cbf818 100644
--- a/src/main/java/graphql/introspection/Introspection.java
+++ b/src/main/java/graphql/introspection/Introspection.java
@@ -784,7 +784,8 @@ public static boolean isIntrospectionTypes(String typeName) {
 
     /**
      * This will look up a field definition by name, and understand that fields like __typename and __schema are special
-     * and take precedence in field resolution
+     * and take precedence in field resolution.  If the parent type is a union type, then the only field allowed
+     * is `__typename`.
      *
      * @param schema     the schema to use
      * @param parentType the type of the parent object
@@ -794,6 +795,43 @@ public static boolean isIntrospectionTypes(String typeName) {
      */
     public static GraphQLFieldDefinition getFieldDef(GraphQLSchema schema, GraphQLCompositeType parentType, String fieldName) {
 
+        GraphQLFieldDefinition fieldDefinition = getSystemFieldDef(schema, parentType, fieldName);
+        if (fieldDefinition != null) {
+            return fieldDefinition;
+        }
+
+        assertTrue(parentType instanceof GraphQLFieldsContainer, "should not happen : parent type must be an object or interface %s", parentType);
+        GraphQLFieldsContainer fieldsContainer = (GraphQLFieldsContainer) parentType;
+        fieldDefinition = schema.getCodeRegistry().getFieldVisibility().getFieldDefinition(fieldsContainer, fieldName);
+        assertTrue(fieldDefinition != null, "Unknown field '%s' for type %s", fieldName, fieldsContainer.getName());
+        return fieldDefinition;
+    }
+
+    /**
+     * This will look up a field definition by name, and understand that fields like __typename and __schema are special
+     * and take precedence in field resolution
+     *
+     * @param schema     the schema to use
+     * @param parentType the type of the parent {@link GraphQLFieldsContainer}
+     * @param fieldName  the field to look up
+     *
+     * @return a field definition otherwise throws an assertion exception if it's null
+     */
+    public static GraphQLFieldDefinition getFieldDefinition(GraphQLSchema schema, GraphQLFieldsContainer parentType, String fieldName) {
+        // this method is optimized to look up the most common case first (type for field) and hence suits the hot path of the execution engine
+        // and as a small benefit does not allocate any assertions unless it completely failed
+        GraphQLFieldDefinition fieldDefinition = schema.getCodeRegistry().getFieldVisibility().getFieldDefinition(parentType, fieldName);
+        if (fieldDefinition == null) {
+            // we look up system fields second because they are less likely to be the field in question
+            fieldDefinition = getSystemFieldDef(schema, parentType, fieldName);
+            if (fieldDefinition == null) {
+                Assert.assertShouldNeverHappen(String.format("Unknown field '%s' for type %s", fieldName, parentType.getName()));
+            }
+        }
+        return fieldDefinition;
+    }
+
+    private static GraphQLFieldDefinition getSystemFieldDef(GraphQLSchema schema, GraphQLCompositeType parentType, String fieldName) {
         if (schema.getQueryType() == parentType) {
             if (fieldName.equals(schema.getIntrospectionSchemaFieldDefinition().getName())) {
                 return schema.getIntrospectionSchemaFieldDefinition();
@@ -805,11 +843,6 @@ public static GraphQLFieldDefinition getFieldDef(GraphQLSchema schema, GraphQLCo
         if (fieldName.equals(schema.getIntrospectionTypenameFieldDefinition().getName())) {
             return schema.getIntrospectionTypenameFieldDefinition();
         }
-
-        assertTrue(parentType instanceof GraphQLFieldsContainer, "Should not happen : parent type must be an object or interface %s", parentType);
-        GraphQLFieldsContainer fieldsContainer = (GraphQLFieldsContainer) parentType;
-        GraphQLFieldDefinition fieldDefinition = schema.getCodeRegistry().getFieldVisibility().getFieldDefinition(fieldsContainer, fieldName);
-        assertTrue(fieldDefinition != null, "Unknown field '%s' for type %s", fieldName, fieldsContainer.getName());
-        return fieldDefinition;
+        return null;
     }
-}
+}
\ No newline at end of file
diff --git a/src/main/java/graphql/normalized/ExecutableNormalizedOperationFactory.java b/src/main/java/graphql/normalized/ExecutableNormalizedOperationFactory.java
index 735ab899d9..7ed78470d5 100644
--- a/src/main/java/graphql/normalized/ExecutableNormalizedOperationFactory.java
+++ b/src/main/java/graphql/normalized/ExecutableNormalizedOperationFactory.java
@@ -34,6 +34,7 @@
 import graphql.schema.FieldCoordinates;
 import graphql.schema.GraphQLCompositeType;
 import graphql.schema.GraphQLFieldDefinition;
+import graphql.schema.GraphQLFieldsContainer;
 import graphql.schema.GraphQLInterfaceType;
 import graphql.schema.GraphQLNamedOutputType;
 import graphql.schema.GraphQLObjectType;
@@ -644,7 +645,7 @@ private ExecutableNormalizedField createNF(CollectedFieldGroup collectedFieldGro
             Set<GraphQLObjectType> objectTypes = collectedFieldGroup.objectTypes;
             field = collectedFieldGroup.fields.iterator().next().field;
             String fieldName = field.getName();
-            GraphQLFieldDefinition fieldDefinition = Introspection.getFieldDef(graphQLSchema, objectTypes.iterator().next(), fieldName);
+            GraphQLFieldDefinition fieldDefinition = Introspection.getFieldDefinition(graphQLSchema, objectTypes.iterator().next(), fieldName);
 
             Map<String, Object> argumentValues = ValuesResolver.getArgumentValues(fieldDefinition.getArguments(), field.getArguments(), CoercedVariables.of(this.coercedVariableValues.toMap()), this.options.graphQLContext, this.options.locale);
             Map<String, NormalizedInputValue> normalizedArgumentValues = null;