Are persistent connections used? How can I find out whether my Drupal 7 site using persistent connection or not? And it's possible to turn it on if it's not?
If the persistent connections is not enabled you can enable it by adding this to settings.php:

ini_set('mysql.allow_persistent', TRUE);


And you just do like this to turn it off:

ini_set('mysql.allow_persistent', TRUE);


To check is persistent connections is enabled go to /admin/reports/status/php, and search for allow_persistent.