最大执行超时会话开始

I am getting a Maximum execution time of 30 seconds exceeded in D:\wamp\www\session_start.php on line 2

I am putting this file above all my scripts. And those randomly getting the error. Can anyone tell what is happening ? And how can fix this ?

<?php
session_start(); //this line gets the error
  if (!isset($_SESSION['user_id'])) {
    if (isset($_COOKIE['user_id']) && isset($_COOKIE['username'])) {
      $_SESSION['user_id'] = $_COOKIE['user_id'];
      $_SESSION['username'] = $_COOKIE['username'];
    }
  }

  ?>