Added performance tip

Author: wouterj

reference/forms/types/choice.rst

@@ -160,19 +160,24 @@ These options inherit from the :doc:`form </reference/forms/types/form>` type:
 Field Variables
 ---------------
 
-====================== ============ ===========================================================
-Variable               Type         Usage                                                      
-====================== ============ ===========================================================
-multiple               ``Boolean``  The value of the `multiple`_ option                        
-expanded               ``Boolean``  The value of the `expanded`_ option                        
-preferred_choices      ``array``    A nested array containing the ``ChoiceView`` objects of    
-                                    choices which should be presented to the user with priority
-choices                ``array``    A nested array containing the ``ChoiceView`` objects of    
-                                    the remaining choices                                      
-separator              ``string``   The seperator to use between choice groups                 
-empty_value            ``mixed``    The empty value if not already in the list, otherwise      
-                                    ``null``                                                   
-is_selected            ``callable`` A callable which takes a ``ChoiceView`` and the value      
-                                    and returns if the choice is selected or not               
-empty_value_in_choices ``Boolean``  Whether the empty value is in the choice list              
-====================== ============ ===========================================================
+====================== ============ ================================================================
+Variable               Type         Usage
+====================== ============ ================================================================
+multiple               ``Boolean``  The value of the `multiple`_ option.
+expanded               ``Boolean``  The value of the `expanded`_ option.
+preferred_choices      ``array``    A nested array containing the ``ChoiceView`` objects of
+                                    choices which should be presented to the user with priority.
+choices                ``array``    A nested array containing the ``ChoiceView`` objects of
+                                    the remaining choices.
+separator              ``string``   The seperator to use between choice groups.
+empty_value            ``mixed``    The empty value if not already in the list, otherwise
+                                    ``null``.
+is_selected            ``callable`` A callable which takes a ``ChoiceView`` and the selected value(s)
+                                    and returns whether the choice is in the selected value(s).
+empty_value_in_choices ``Boolean``  Whether the empty value is in the choice list.
+====================== ============ ================================================================
+
+.. tip::
+
+    It's significantly faster to use the :ref:`form-twig-selectedchoice` test
+    instead when using Twig.