OPTIONS takes exactly one minute

Certain users are seeing a 1 minute delay on commits. Whilst it isn't certain, so far it appears that its only users using WEBDAV authentication. I'm not sure what "OPTIONS" does so I guess that's what I want to know so I can track down where this delay is occurring.
This is the log of a user with a 1 minute delay. The user not using WEBDAV sees the same entries but the OPTIONS is almost instant.

<server ip> - - [19/Jul/2013:08:40:29 +0100] "OPTIONS /svn/engineering/branches/Libs HTTP/1.1" 401 381 0
<server ip> - paul.hester [19/Jul/2013:08:40:29 +0100] "OPTIONS /svn/engineering/branches/Libs HTTP/1.1" 200 150 60
<server ip> - paul.hester [19/Jul/2013:08:41:29 +0100] "POST /svn/engineering/!svn/me HTTP/1.1" 201 - 0

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Received on 2013-07-19 10:45:39 CEST