diff --git a/lib/simpletest/testhtmlpurifier.php b/lib/simpletest/testhtmlpurifier.php
index b138becfb77..65725cbce29 100644
--- a/lib/simpletest/testhtmlpurifier.php
+++ b/lib/simpletest/testhtmlpurifier.php
@@ -193,6 +193,63 @@ class htmlpurifier_test extends UnitTestCase {
         $text = 'x<form></form>x';
         $this->assertIdentical('xx', purify_html($text));
     }
+
+    /**
+     * Test internal function used for clean_text() speedup.
+     */
+    function test_is_purify_html_necessary() {
+        // first our shortcuts
+        $text = "";
+        $this->assertFalse(is_purify_html_necessary($text));
+        $this->assertidentical($text, purify_html($text));
+
+        $text = "666";
+        $this->assertFalse(is_purify_html_necessary($text));
+        $this->assertidentical($text, purify_html($text));
+
+        $text = "abc\ndef \" ' ";
+        $this->assertFalse(is_purify_html_necessary($text));
+        $this->assertidentical($text, purify_html($text));
+
+        $text = "abc\n<p>def</p>efg<p>hij</p>";
+        $this->assertFalse(is_purify_html_necessary($text));
+        $this->assertidentical($text, purify_html($text));
+
+        $text = "<br />abc\n<p>def<em>efg</em><strong>hi<br />j</strong></p>";
+        $this->assertFalse(is_purify_html_necessary($text));
+        $this->assertidentical($text, purify_html($text));
+
+        // now failures
+        $text = "&nbsp;";
+        $this->assertTrue(is_purify_html_necessary($text));
+
+        $text = "Gin & Tonic";
+        $this->assertTrue(is_purify_html_necessary($text));
+
+        $text = "Gin > Tonic";
+        $this->assertTrue(is_purify_html_necessary($text));
+
+        $text = "Gin < Tonic";
+        $this->assertTrue(is_purify_html_necessary($text));
+
+        $text = "<div>abc</div>";
+        $this->assertTrue(is_purify_html_necessary($text));
+
+        $text = "<span>abc</span>";
+        $this->assertTrue(is_purify_html_necessary($text));
+
+        $text = "<br>abc";
+        $this->assertTrue(is_purify_html_necessary($text));
+
+        $text = "<p class='xxx'>abc</p>";
+        $this->assertTrue(is_purify_html_necessary($text));
+
+        $text = "<p>abc<em></p></em>";
+        $this->assertTrue(is_purify_html_necessary($text));
+
+        $text = "<p>abc";
+        $this->assertTrue(is_purify_html_necessary($text));
+
+    }
 }
 
-
diff --git a/lib/weblib.php b/lib/weblib.php
index 22141d27b4e..feb615bcf8d 100644
--- a/lib/weblib.php
+++ b/lib/weblib.php
@@ -1450,9 +1450,7 @@ function trusttext_active() {
  * @return string The cleaned up text
  */
 function clean_text($text, $format = FORMAT_HTML, $options = array()) {
-    if (empty($text) or is_numeric($text)) {
-       return (string)$text;
-    }
+    $text = (string)$text;
 
     if ($format != FORMAT_HTML and $format != FORMAT_HTML) {
         // TODO: we need to standardise cleanup of text when loading it into editor first
@@ -1463,7 +1461,9 @@ function clean_text($text, $format = FORMAT_HTML, $options = array()) {
         return $text;
     }
 
-    $text = purify_html($text, $options);
+    if (is_purify_html_necessary($text)) {
+        $text = purify_html($text, $options);
+    }
 
     // Originally we tried to neutralise some script events here, it was a wrong approach because
     // it was trivial to work around that (for example using style based XSS exploits).
@@ -1473,6 +1473,53 @@ function clean_text($text, $format = FORMAT_HTML, $options = array()) {
     return $text;
 }
 
+/**
+ * Is it necessary to use HTMLPurifier?
+ * @private
+ * @param string $text
+ * @return bool false means html is safe and valid, true means use HTMLPurifier
+ */
+function is_purify_html_necessary($text) {
+    if ($text === '') {
+        return false;
+    }
+
+    if ($text === (string)((int)$text)) {
+        return false;
+    }
+
+    if (strpos($text, '&') !== false or preg_match('|<[^pesb/]|', $text)) {
+        // we need to normalise entities or other tags except p, em, strong and br present
+        return true;
+    }
+
+    $altered = htmlspecialchars($text, ENT_NOQUOTES, 'UTF-8', true);
+    if ($altered === $text) {
+        // no < > or other special chars means this must be safe
+        return false;
+    }
+
+    // let's try to convert back some safe html tags
+    $altered = preg_replace('|&lt;p&gt;(.*?)&lt;/p&gt;|m', '<p>$1</p>', $altered);
+    if ($altered === $text) {
+        return false;
+    }
+    $altered = preg_replace('|&lt;em&gt;([^<>]+?)&lt;/em&gt;|m', '<em>$1</em>', $altered);
+    if ($altered === $text) {
+        return false;
+    }
+    $altered = preg_replace('|&lt;strong&gt;([^<>]+?)&lt;/strong&gt;|m', '<strong>$1</strong>', $altered);
+    if ($altered === $text) {
+        return false;
+    }
+    $altered = str_replace('&lt;br /&gt;', '<br />', $altered);
+    if ($altered === $text) {
+        return false;
+    }
+
+    return true;
+}
+
 /**
  * KSES replacement cleaning function - uses HTML Purifier.
  *