Login via Shibboleth Very Slow

Using the conventional Drupal login, it takes about 10 seconds from pressing submit to being logged in when the site is busy: a live site running very slowly, alas.

Using the Shibboleth login it's taking around 60 seconds to turn around. Our IdP is sending back the results of the attribute query in about 250ms, then the IdP POST is made to http://www.site.edu/Shibboleth.sso/SAML/POST. It's at this point that logging in hangs for almost a minute.

I've had a look through the shib_auth.module code and it all seems pretty basic: it essentially checks the HTTP headers exist, sees if the user already exists (to either login or create a new user), and checks/updates the email address as required. I currently have no rules defined so have ignored all the related functions.

Is there anything else that could be slowing things down? Or is this a comparable performance to other sites? Just wanted to check as I'm about to pull apart shibboleth2.xml and related attribute files.

On campus, other systems using Shibboleth (such as Moodle) usually turn around the login cycle in about 10 seconds so I just wonder what I've done wrong.