LINQ to Datatable is very slow

I'm developing a web page for daily reports for the organization I work. I'm using ASP.NET MVC with SQL DATABASE, first I do is to create stored procedures then load them into Model like this:

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Then I created a method in controller that will receive some parameters from view to send it again to the stored procedure function in the model, inside stored procedure model I used LINQ to group data and load it in LIST then return it to controller method, after that the controller method will be invoked by ajax to return JSON file, then JSON file will be loaded into DATATABLE.

I create 8 reports using this way and it was very fast, but in the last report, that contains 50 columns and 1K rows, it takes at least 110 seconds to load data into DATATABLE, even it takes less than 2s to load past reports which contain less columns and more rows.