diff --git a/src/markup/syntax/highlighter/PhutilPygmentsSyntaxHighlighter.php b/src/markup/syntax/highlighter/PhutilPygmentsSyntaxHighlighter.php
--- a/src/markup/syntax/highlighter/PhutilPygmentsSyntaxHighlighter.php
+++ b/src/markup/syntax/highlighter/PhutilPygmentsSyntaxHighlighter.php
@@ -2,6 +2,9 @@
 
 final class PhutilPygmentsSyntaxHighlighter extends Phobject {
 
+  const MAX_LINE_LENGTH = 10240;
+  const MAX_SOURCE_LENGTH = 1048576;
+
   private $config = array();
 
   public function setConfig($key, $value) {
@@ -17,6 +20,12 @@
       // use it on files with "\r" newlines. If we have "\r" not followed by
       // "\n" in the file, skip highlighting.
       $language = null;
+    } else if ((strlen($source) > self::MAX_SOURCE_LENGTH) ||
+        ($this->getLongestLineLength($source) > self::MAX_LINE_LENGTH)) {
+      // `pygmentize` is really slow and expensive when dealing with long
+      // source but especially with long lines. Better skip highlighting than
+      // 500 error.
+      $language = null;
     }
 
     if ($language) {
@@ -40,6 +49,17 @@
       ->getHighlightFuture($source);
   }
 
+  private function getLongestLineLength($source) {
+    $longest = 0;
+    $offset = 0;
+    while (($pos = strpos($source, "\n", $offset)) !== false) {
+      $longest = max($longest, $pos - $offset);
+      $offset = $pos + 1;
+    }
+    $longest = max($longest, strlen($source) - $offset);
+    return $longest;
+  }
+
   private function getPygmentsLexerNameFromLanguageName($language) {
     static $map = array(
       'adb' => 'ada',