To answer your questions:
- I am assuming the Laharsub server is running on a different machine than the XP client. Correct? - Correct
- Are there any differences in the browser model or version between Win7 and XP machines? - No (I used Firefox, IE8 and chrome for both machines in order to test them)
- Is there any difference in the Silverlight version between the two machines? - No, Both machines are running SL 4
- Are web proxies configured in the same way in Win7 and XP machines? - No
- Is Fiddler2 providing any insight into what's going on at the HTTP level?
- If 1-5 does not provide any interesting insight, I would try looking at TCP level traffic using NetMon from http://www.microsoft.com/downloads/en/details.aspx?FamilyID=983b941d-06cb-4658-b7f6-3088333d062f.
Regarding questions 5 & 6:
I tried to use the jquery chat client in the xp machine to check if messages arrive and their latency (because fiddler showed that they did).
The way I did it is that I subscribed with the chat client to the same topic as the drawing client and saw when binary messages arrive to the chat client.
I was surprised (and happy) to see that the performance with the jquery client on the XP machine was really good.
This leads me to the conclusion that the problem is with the Silverlight client - as odd as it may seem, the SL clients behave differently on different operating systems.
I will investigate further and let you know what I find.
Let me know if you find anything as well or if you can think of a reason for this problem.