Re-applied an optimization that existed in the previous implementation.

String results will not be "casted" using the identity function found
in the StringType class, as a way of sparing some function calls.

src/Database/FieldTypeConverter.php

@@ -14,6 +14,8 @@
  */
 namespace Cake\Database;
 
+use Cake\Database\Type\OptionalConvertInterface;
+
 /**
  * A callable class to be used for processing each of the rows in a statement
  * result, so that the values are converted to the right PHP types.
@@ -49,6 +51,12 @@ public function __construct(TypeMap $typeMap, Driver $driver)
         $types = array_map(['Cake\Database\Type', 'build'], array_combine($types, $types));
         $result = [];
 
+        foreach ($types as $k => $type) {
+            if ($type instanceof OptionalConvertInterface && !$type->requiresToPHPCast()) {
+                unset($types[$k]);
+            }
+        }
+
         foreach ($map as $field => $type) {
             if (isset($types[$type])) {
                 $result[$field] = $types[$type];

src/Database/Type/OptionalConvertInterface.php

@@ -0,0 +1,31 @@
+<?php
+/**
+ * CakePHP(tm) : Rapid Development Framework (http://cakephp.org)
+ * Copyright (c) Cake Software Foundation, Inc. (http://cakefoundation.org)
+ *
+ * Licensed under The MIT License
+ * For full copyright and license information, please see the LICENSE.txt
+ * Redistributions of files must retain the above copyright notice.
+ *
+ * @copyright     Copyright (c) Cake Software Foundation, Inc. (http://cakefoundation.org)
+ * @link          http://cakephp.org CakePHP(tm) Project
+ * @since         3.2.0
+ * @license       http://www.opensource.org/licenses/mit-license.php MIT License
+ */
+namespace Cake\Database\Type;
+
+/**
+ * An interface used by Type objects to signal whether the casting
+ * is actually required.
+ */
+interface OptionalConvertInterface
+{
+
+    /**
+     * Returns whehter the cast to PHP is required to be invoked, since
+     * it is not a indentity function.
+     *
+     * @return boolean
+     */
+    public function requiresToPHPCast();
+}

src/Database/Type/StringType.php

@@ -24,7 +24,7 @@
  *
  * Use to convert string data between PHP and the database types.
  */
-class StringType extends Type
+class StringType extends Type implements OptionalConvertInterface
 {
 
     /**
@@ -94,4 +94,14 @@ public function marshal($value)
         }
         return (string)$value;
     }
+
+    /**
+     * {@inheritDoc}
+     *
+     * @return boolean False as databse results are returned already as strings
+     */
+    public function requiresToPHPCast()
+    {
+        return false;
+    }
 }