diff --git a/includes/theme.inc b/includes/theme.inc
index 8d5348d..1d70f0a 100644
--- a/includes/theme.inc
+++ b/includes/theme.inc
@@ -1237,17 +1237,17 @@ function path_to_theme() {
 /**
  * Allows themes and/or theme engines to discover overridden theme functions.
  *
- * @param $cache
+ * @param array $cache
  *   The existing cache of theme hooks to test against.
- * @param $prefixes
+ * @param array $prefixes
  *   An array of prefixes to test, in reverse order of importance.
  *
- * @return $implementations
+ * @return array
  *   The functions found, suitable for returning from hook_theme;
  */
 function drupal_find_theme_functions($cache, $prefixes) {
   $implementations = array();
-  $functions = get_defined_functions();
+  $grouped_functions = drupal_group_functions_by_prefix();
 
   foreach ($cache as $hook => $info) {
     foreach ($prefixes as $prefix) {
@@ -1263,8 +1263,10 @@ function drupal_find_theme_functions($cache, $prefixes) {
       // are found using the base hook's pattern, not a pattern from an
       // intermediary suggestion.
       $pattern = isset($info['pattern']) ? $info['pattern'] : ($hook . '__');
-      if (!isset($info['base hook']) && !empty($pattern)) {
-        $matches = preg_grep('/^' . $prefix . '_' . $pattern . '/', $functions['user']);
+      // Grep only the functions which are within the prefix group.
+      list($first_prefix,) = explode('_', $prefix, 2);
+      if (!isset($info['base hook']) && !empty($pattern) && isset($grouped_functions[$first_prefix])) {
+        $matches = preg_grep('/^' . $prefix . '_' . $pattern . '/', $grouped_functions[$first_prefix]);
         if ($matches) {
           foreach ($matches as $match) {
             $new_hook = substr($match, strlen($prefix) + 1);
@@ -1292,6 +1294,25 @@ function drupal_find_theme_functions($cache, $prefixes) {
 }
 
 /**
+ * Group all user functions by word before first underscore.
+ *
+ * @return array
+ *   Functions grouped by the first prefix.
+ */
+function drupal_group_functions_by_prefix() {
+  $functions = get_defined_functions();
+
+  $grouped_functions = array();
+  // Splitting user defined functions into groups by the first prefix.
+  foreach ($functions['user'] as $function) {
+    list($first_prefix,) = explode('_', $function, 2);
+    $grouped_functions[$first_prefix][] = $function;
+  }
+
+  return $grouped_functions;
+}
+
+/**
  * Allows themes and/or theme engines to easily discover overridden templates.
  *
  * @param $cache