I can reproduce this waiting time only with a fresh chrome session. Edge and Firfox is working fine. Looking into Chrome's dev tools (performance tab) declared most of the waiting time as idle.
The client is just waiting:
Accordingly you'll need to check the server side, which we can't do without code / server access.
The only difference (I can spot) between the slow and the fast chrome session is, that the slow session complains about a missing favicon.ico:
GET https://www.zmbp-resources.uni-tuebingen.de/favicon.ico 404 (Not Found)
This is no longer the case in the fast sessions - see your browsers console. Maybe this GET times out the first time the page is accessed.
This article provides a possible explanation:
What happens in these cases is that upon a file not being found, the website serves an HTML page announcing that the file was not found. In these cases, the html is heavy with inline CSS, JavaScript, etc.
When images, scripts or other files fail to load, a simple HTTP 404 response should be served, and the large 404 html page should only be delivered when a webpage does not load properly. To serve a huge 404 page (or to just load the root page of your site) wastes data and server processing.