ASP.NET MVC and Ajax, concurrent requests?

AJAX newbie here!
At the moment in my ASP.NET MVC web app my AJAX requests appear to be getting batched or queued, im not sure.
No requests seem to be getting completed until the previous request has finished.
How do I go about getting the requests to return independantly?
I dont necessarily want someone to give me the answer but maybe some links to good tutorials or resources which could help. Thanks

I'm expanding on Lachlan Roche's answer, which is correct.

The ASP.NET framework will "single-thread" requests that deal with Session scope (a global resource), to prevent one request from interfering with another. In WebForms I think you can use the Page directive to specify that individual pages don't use Session and therefore don't need to treated synchronously like this.

The problem is that in ASP.NET MVC all requests use Session, because it's used to implement TempData. You can disable session state entirely, as Lachlan Roche pointed out, or you can deal with this on a case-by-case basis.

A possible solution might be to kick off your own background threads to process any long-running code, so that the initial request "completes" as quickly as possible.

ASP.NET will serially process requests on a per-session basis unless sessions are configured as disabled or read only in web.config via the enableSessionState attribute on the pages element.

As this is a page setting, this will not affect MVC controllers and they will still be subject to serial request processing.

Curiously, even with sessions disabled or set to readonly, we can still read and write session data. It seems to only affect the session locking that causes serial request processing.

<system.web>
    <pages enableSessionState="ReadOnly"/>
</system.web>

Pages can also have an enableSessionState property, though this is not relevant to MVC views.

<%@ Page EnableSessionState="True" %>