IIS6/NTLM : very slow authentication with user from other domain

I have a WSS 3.0 website hosted on IIS6. It's fully configured for NTLM only.

It is hosted in domain A. When users from domain A attempt to access the website, it works fine.

Now we are in the process of migration user from domain A to domain B (different forest, with external bidirectional trust).
– Users in domain B leverage SIDHistory from domain A
– For this specific website, WSS site has been configured to explicitly authorize Domain A Users and Domain B Users to access the website

Here are the different scenario…

1. Windows XP / IE7 or IE8 / User from A –> access OK
2. Windows XP / IE7 or IE8 / User from B –> access is very slow because of authentication
   • I can see the usual NTLM exchange with HTTP 401 answer but it takes more than 10 seconds to the server to final send the answer on the final exchange
3. Windows XP / IE7 or IE8 / User from B + disable "Windows Integrated Authentication" in IE –> access is ok this time
   • HTTP authentication sequence looks strictly similar but this time the server provides the final answer immediatly
4. Windows 7 / IE9 / User from B / Windows Integrated Authentication enabled –> Access OK

Any idea to explain the slow access for case #2, and why the test with case #3 resolves the problem (just a test, I don't want to disable this option)

Regards.