If both Basic and Windows authentication is turned-on for the Exchange virtual directory on the Exchange server, and WebDAV code tries to access the Exchange box through a firewall having port 88 blocked, you will get a long delay.
The reason for the delay is that a Basic call will first attempt Anonymous communication first. When it fails, it uses the response it got back from the server to determine the next attempt. A failed Anonymous check will respond with the protocols supported by the virtual directory. Since the virtual directory supports Windows authentication, the client WebDAV call will try to do authentication against the DC it was told about with the response of the Anonymous check... this is the delay... After the delay, Basic Authentication is then performed - which will work... The credentials are then cached for a short period - which is why additional WebDAV calls done right after the first one will authenticate quickly.
If this is the scenario, the following are the options available:
1) Open port 88
2) Have ONLY Basic turned on for the Exchange virtual directory.3) Create a secondary virtual directory which has the same settings as the Exchange virtual directory; however you will need to be sure that the only difference is that Basic ONLY is enabled on this new virtual directory. One idea is to call this new virtual directory “ExchangeBasic” or perhaps “Exchange2”.