HttpRequest to external system blocks internal requests

Hello all,

I created a small application, which gets / stores data in a database, and display it in an overview and details page.
This application also needs data from an external system for the details view, which I can get by REST-API.

When I now open the details page, and my backend requests data from the external system by HttpRequest (which may take serveral seconds up to minutes), I cannot open a new overview page in another browser. The system is blocked until the HttpRequest to the external system is processed.
The internal requests, and the request to the external system have their own HttpController.

How can I fix this issue?