Domino 7 / IIS 6 performance slow down

Users are authenticated to our Domino web application via IIS (port 80). The application can also be accessed via Domino Web Server on port 81.

Over the past week, when accessing the application via IIS/port 80 it is extremely slow to the point of being unusable. It is very evident that it is taking a log time to load icon images etc. However, when accessing the application via Domino/port 81, it is fine.

Does anyone have any advice on how to troubleshoot this or what to look for? Any help would be greatly appreciated.

Thanks.