ASP.NET Server does not process pages asynchronously
Solution 1:
This is happens because the session locks the page reads.
So when the one page is loading, the session lock all the rest request until is finish and send the page.
To make it work you need ether to disabled the session on this pages, ether use handler that is not have by default session lock.
Relative questions :
Trying to make Web Method Asynchronous
Web app blocked while processing another web app on sharing same session
What perfmon counters are useful for identifying ASP.NET bottlenecks?
Replacing ASP.Net's session entirely