Unit Testing for concurrent requests in web application

I am working on a web application which is accessed by many concurrent users. I am using MVC 4 and my action method in a controller will call a manager class and get the data from database using Entity Framework. Also, some times the data will be refined using LINQ and send back the result to Action method. This whole process takes 2-3 seconds, and within this time one or more requests can come before the first one finish it.

Now, I need to write a unit test to simulate the above test case and confirm that my code is working.

Please suggest, if any improvement in the application design for concurrent users case.