Question: My Flask applications has to do quite a large calculation to fetch a certain page. While Flask is doing that function, another user cannot access the website, because Flask is busy with the large calculation.

Is there any way that I can make my Flask application accept requests from multiple users?

Answer: 
Yes, deploy your application on a different WSGI server, see the [Flask deployment options documentation](http://flask.pocoo.org/docs/deploying/).

The server component that comes with Flask is really only meant for when you are developing your application; even though it can be configured to handle concurrent requests with `app.run(threaded=True)` (as of Flask 1.0 this is the default). The above document lists several options for servers that can handle concurrent requests *and* are far more robust and tuneable.
For requests that take a long time, you might want to consider starting a background job for them.