To verify the issue further and to make sure the issue is not due to your server setup or config ( especially since you said using different laptop/devices yields different results which points to some connection or server issues ) will it be possible for you to setup an exact copy/clone site in a different server and see how the load speed for the pages goes there?
I deactivated all plugins except WPML and activated a default theme Twenty Twenty-Three. I then went to the Seiten/Page page on the site.
hidden link
The page loads fine in less than 0.3 seconds indifferent of weather I activate or deactivate WPML.
So the issue does not seems to be with WPML specifically. I recommend you to test the issue further on your main site, you can run the conflict test as you did initially or co-ordinate with the server provider further to see if it helps.