With a delay as long as that I think that there's a timeout involved. For example: some connection is being attempted but there's no response so the call waits and waits until the timeout expires and then carries on. Maybe it's a network connection to a website, or some service like a database.
What additional plugins are installed? See these under Site administration ▸ Plugins ▸ Plugins overview (or go to admin/plugins.php to avoid the 5 minute wait) and click Additional plugins to see only these.
Also try enabling debugging (admin/settings.php?section=debugging) by setting:
- Debug message: DEVELOPER
- Display debug messages: Yes.
Then access the slow page and see if any errors appear on screen, and also check the web server error log.