Index: includes/common.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/common.inc,v
retrieving revision 1.1040
diff -u -p -r1.1040 common.inc
--- includes/common.inc	3 Nov 2009 06:47:22 -0000	1.1040
+++ includes/common.inc	3 Nov 2009 22:17:30 -0000
@@ -2683,6 +2683,7 @@ function drupal_attributes(array $attrib
  */
 function l($text, $path, array $options = array()) {
   global $language_url;
+  static $must_theme = NULL;
 
   // Merge in defaults.
   $options += array(
@@ -2702,6 +2703,34 @@ function l($text, $path, array $options 
     $options['attributes']['title'] = strip_tags($options['attributes']['title']);
   }
 
+  // Determine if rendering of the link is to be done with a theme function
+  // or the inline default. Inline is faster, but if the theme system has been
+  // loaded and a module or theme implements a preprocess or process function
+  // or overrides the theme_link() function, then invoke theme(). Preliminary
+  // benchmarks indicate that invoking theme() can slow down the l() function
+  // by 20% or more, and that some of the link-heavy Drupal pages spend more
+  // than 10% of the total page request time in the l() function.
+  if (!isset($must_theme) && function_exists('theme')) {
+    // Allow edge cases to prevent theme initialization and force inline link
+    // rendering.
+    if (variable_get('themeable_links', TRUE)) {
+      drupal_theme_initialize();
+      $registry = theme_get_registry();
+      $must_theme = (
+        !empty($registry['link']['preprocess functions']) ||
+        !empty($registry['link']['process functions']) ||
+        !empty($registry['link']['includes']) ||
+        !isset($registry['link']['function']) ||
+        ($registry['link']['function'] != 'theme_link')
+      );
+    }
+    else {
+      $must_theme = FALSE;
+    }
+  }
+  if ($must_theme) {
+    return theme('link', array('text' => $text, 'path' => $path, 'options' => $options));
+  }
   return '<a href="' . check_plain(url($path, $options)) . '"' . drupal_attributes($options['attributes']) . '>' . ($options['html'] ? $text : check_plain($text)) . '</a>';
 }
 
@@ -5335,6 +5364,9 @@ function drupal_common_theme() {
     'status_messages' => array(
       'variables' => array('display' => NULL),
     ),
+    'link' => array(
+      'variables' => array('text' => NULL, 'path' => NULL, 'options' => array()),
+    ),
     'links' => array(
       'variables' => array('links' => NULL, 'attributes' => array('class' => array('links')), 'heading' => array()),
     ),
Index: includes/theme.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/theme.inc,v
retrieving revision 1.547
diff -u -p -r1.547 theme.inc
--- includes/theme.inc	3 Nov 2009 06:47:22 -0000	1.547
+++ includes/theme.inc	3 Nov 2009 22:17:30 -0000
@@ -1367,6 +1367,34 @@ function theme_status_messages($variable
 }
 
 /**
+ * Return a themed link.
+ *
+ * All Drupal code that outputs a link should call the l() function. That
+ * function performs some initial preprocessing, and then, if necessary,
+ * calls theme('link') for rendering the anchor tag.
+ *
+ * To optimize performance for sites that don't need custom theming of links,
+ * the l() function includes an inline copy of this function, and uses that copy
+ * if none of the enabled modules or the active theme implement any preprocess
+ * or process functions or override this theme implementation.
+ *
+ * @param $variables
+ *   An associative array containing the keys 'text', 'path', and 'options'.
+ *   See the l() function for information about these variables.
+ *
+ * @return
+ *   An HTML string containing a link to the given path.
+ *
+ * @see l()
+ */
+function theme_link($variables) {
+  $text = $variables['text'];
+  $path = $variables['path'];
+  $options = $variables['options'];
+  return '<a href="' . check_plain(url($path, $options)) . '"' . drupal_attributes($options['attributes']) . '>' . ($options['html'] ? $text : check_plain($text)) . '</a>';
+}
+
+/**
  * Return a themed set of links.
  *
  * @param $variables