What's wrong with customer's production environment?

Recently, the system which developed by us has been deployed to customer's production environment(using IE6/7/8 at client):

• Windows 2003 Enterprise Edition SP2
• ASP.NET 3.5 (with ajax f开发者_开发百科unctionality)

When we access the site entry point which is named Default.aspx from the WAN internet connection, it works just fine. However when we access the site from the LAN connection pages take a long time to render.

It shows us a blank page with loading information on the IE's status bar. Whilst it is loading I viewed the source code three times by right clicking on the page and saw the contents increased each time which means that the page is indeed loading.

We know that the connection speed in customer's LAN environment is slow. However, we have to try and resolve this.

So, could anybody give me some tips on how to improve this?

Update:

In reply to your comments.

To Michael: Yes, we all think it should be a network problem, but can't find out where could we go.

To tsegay: Yes, it just slow from local network which have LAN internet connection. But there is no problem in our lan intranet test/development environment.

If it is a LAN security problem, then you would need to isolate an HTTP traffic issue, so IT people can fix. Install Fiddler on the client and check which session(s) is slow. Is it Default.aspx or a resource (if any)? There may be a java script file or an objects (i.e. active-X) on the page that firewall/virus-scan do not like. You will see where the traffic got stuck or crawling.

Security scan can change HTML. Compare in Fiddler content length of Default.aspx accessed via LAN and WAN to make sure that it is not altered. Also, if the problem is with DNS, Fiddler will show high DNS time.