On phpbb.com we had Jabber notifications enabled and recently we started seeing PHP time limits being reached.
Fatal error: Maximum execution time of 30 seconds exceeded in /community/includes/functions_jabber.php on line 304
The timeout happens while sending notifications as well as during the ACP's login test when you submit the Jabber settings page.
Something else I noticed is that the default timeout is 10 seconds for sending a Jabber message, which means we can have most 2 users requesting notifications with a faulty server before going past the default PHP timeout of 30 seconds. It may be useful to have an overall timeout value for all pending notifications.