AWStats and Time Taken to serve the Request
Solution 1:
It'll be very difficult to know how long the entire page takes to load. Not impossible, but difficult. You will need 'group' by referrals.
For each PHP/ASP/Whatever/HTML page that is output, then find all the requests immediately after it that have the referrer of the just-requested page, for that IP address. Keep going until that IP requests another HTML page. Repeat and rinse.
As for how to actually do this in AWStats, I honestly don't know. You might need to do up a script or something to parse the logs manually.
Solution 2:
Here's a hack that will probably do what you want: http://www.pgs-soft.com/installing-awstats-on-iis-7-0-time-taken-extra-section.html
Solution 3:
It really depends on the question that you're trying to answer. Is is:
"Our servers are overloaded, find the offending pages,"
in which case you'd be doing something with the IIS logging. On the other hand, you may be trying to answer:
"Which pages are the slowest to load from a user perspetive,"
in which case AWStats isn't going to help you, you'd have to use an analysis tool, probably running outside your network.