Loading page in ASP.NET MVC while procesing

Question

I have problem with loading page in my ASP.NET MVC app. I would like to show user that something is processing in the background.

When the user is loading a .csv files with ~100k records into the database, it is taking like 15 minutes. In the meantime, the user only sees a blank page with a loading spinner in browser tab.

I tried to use a lot of examples of loading pages, but all of them are running on page start and constant running time. In my app, everything is processed first, then page is loaded.

I'm looking for solution that shows user loading spinner/page (it doesn't matter) but user would see that app is working on his task. I thought that I could do it when button is clicked, but this solution won't work for me, because I would like to open all user task in new browser tab to let user compare a few request easier.