diff -u b/core/lib/Drupal/Core/Theme/Registry.php b/core/lib/Drupal/Core/Theme/Registry.php
--- b/core/lib/Drupal/Core/Theme/Registry.php
+++ b/core/lib/Drupal/Core/Theme/Registry.php
@@ -138,6 +138,13 @@
   protected $themeManager;
 
   /**
+   * User functions grouped by the word before first underscore.
+   *
+   * @var array
+   */
+  protected $groupedFunctions;
+
+  /**
    * Constructs a \Drupal\Core\Theme\Registry object.
    *
    * @param string $root
@@ -573,7 +580,7 @@
    * @see ::processExtension()
    */
   protected function postProcessExtension(array &$cache, ActiveTheme $theme) {
-    $grouped_functions = drupal_group_functions_by_prefix();
+    $grouped_functions = $this->getPrefixGroupedUserFunctions();
 
     // Gather prefixes. This will be used to limit the found functions to the
     // expected naming conventions.
@@ -683,6 +690,28 @@
   }
 
   /**
+   * Get all user functions grouped by word before first underscore.
+   *
+   * @return array
+   *   Functions grouped by the first prefix.
+   */
+  public function getPrefixGroupedUserFunctions() {
+    if (!$this->groupedFunctions) {
+      $functions = get_defined_functions();
+
+      $this->groupedFunctions = [];
+      // Splitting user defined functions into groups by the first prefix.
+      foreach ($functions['user'] as $function) {
+        list($first_prefix,) = explode('_', $function, 2);
+        $this->groupedFunctions[$first_prefix][] = $function;
+      }
+    }
+
+    return $this->groupedFunctions;
+  }
+
+
+  /**
    * Wraps drupal_get_path().
    *
    * @param string $module
only in patch2:
unchanged:
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -172,16 +172,7 @@ function drupal_find_theme_functions($cache, $prefixes) {
  *   Functions grouped by the first prefix.
  */
 function drupal_group_functions_by_prefix() {
-  $functions = get_defined_functions();
-
-  $grouped_functions = [];
-  // Splitting user defined functions into groups by the first prefix.
-  foreach ($functions['user'] as $function) {
-    list($first_prefix,) = explode('_', $function, 2);
-    $grouped_functions[$first_prefix][] = $function;
-  }
-
-  return $grouped_functions;
+  \Drupal::service('theme.registry')->getPrefixGroupedUserFunctions();
 }
 
 /**