I was unhappy with the performance of our Drupal websites when using them logged in. We have a local test machine that is LAMP (Linux, Apahce, MySQL, PHP) and it was had page load times of over a second quicker than the production server that is WIMP (Windows, IIS, MySQL, PHP). While the machine are using different OS and Webservers that didn’t account for the issues. Using the Devel module it was telling me the MySQL queries were almost identical between the two machines but the page execution time was significantly slower on the production machine.
After using a profiling extension for PHP I tracked down the problem. PDO was taking over a second (!) to connect to the MySQL database on localhost. After much searching around it turns out the problem seems to be due to MySQL having issues with resolving names with IPv6 addresses. Windows Server 2008 R2 by default returns an IPv6 (::1) and IPv4 (127.0.0.1) address for localhost. In order to work around the issue I needed to add ‘127.0.0.1 localhost’ to the servers \Windows\System32\etc\hosts file. It should be noted for anyone else doing the same, make sure there is no IPv6 address for localhost in the hosts file either.
After making the change, database connections were instant and the performance of the sites improved considerably.