IIS 7.5 Max Concurrent requests per system

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I hosted an ASP.NET MVC5 web application on a windows server 2008 R2 SP1. in this website there is a upload file section which users can upload large files with max size of 1 GB. When they click on upload button another popup window will be opened to allow users do other things during files are being uploaded. I used MvcFileUploader for upload. The problem is while upload process is working, they can't open other pages from the site. I have tested theses items when files are uploading:

Users can open other websites ( result is uploader isn't using the whole available network traffic)
While a user is uploading, other users from another system can open our website easily (result is IIS server is working and isn't not responding)
So I came up with this conclusion that somewhere should be a restriction for max concurrent connection from each IP/system. Am I right? do you have any suggestions? I have already changed MvcFileUploader default configuration with these:

limitConcurrentUploads: 1,
sequentialUploads: true,