Performance overhead issue

In our environment we are experiencing serious performance issue when the traffic is routed through F5. Directly accessing web server has no issue. Traffic through F5 taking 10 times more time than directly accessing web server. We observed this as end-user. Further analysis showed that there is file (or) script name looks like "xyz.policy" is responsible for high response time. "xyz.policy" was configured to enforce security for accessing files on the web server. Accessing a file for example "MyFile.html" will redirect to and demands user credentials. From the IIS logs we can see that web server side "time-taken" stats is few milliseconds but overall end-user response time is 40+ seconds.

Now I would like to ask our F5 admin to help us analysing this performance issue. I need your help in asking right questions to the F5 admin. I do not have access to F5. Please also suggest how to diagnose such performance issues w.r.t F5.

Regards, Prasad