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
@@ -570,10 +570,7 @@
    *   Current active theme.
    */
   protected function postProcessExtension(array &$cache, $theme) {
-
-    // Get all user defined functions.
-    list(, $user_func) = array_values(get_defined_functions());
-    $user_func = array_combine($user_func, $user_func);
+    $grouped_functions = drupal_group_functions_by_prefix();
 
     // Gather prefixes. This will be used to limit the found functions to the
     // expected naming conventions.
@@ -593,7 +590,11 @@
     // Collect all variable processor functions in the correct order.
     $processors = [];
     foreach ($prefixes as $prefix) {
-      $processors += preg_grep("/^{$prefix}_preprocess_($hooks)(__)?/", $user_func);
+      // Grep only the functions which are within the prefix group.
+      list($first_prefix,) = explode('_', $prefix, 2);
+      if (isset($grouped_functions[$first_prefix])) {
+        $processors += preg_grep("/^{$prefix}_preprocess_($hooks)(__)?/", $grouped_functions[$first_prefix]);
+      }
     }
 
     // Add missing variable processors. This is needed for hooks that do not