It appears that Symfony2 is waiting for AJAX response after a request. It will not go to another link on the same page until the response comes back.
This article d
Just a warning for those who are using the PHP built-in web server (as I got trapped, it might help others):
From the PHP doc:
The web server runs a only one single-threaded process, so PHP applications will stall if a request is blocked.
It means that even if you close the session properly, you'll still end up stailing with one connection at once.
http://php.net/manual/en/features.commandline.webserver.php