I started debugging roundcube, and so far, it looks like the carddav-plugin is causing the timeout.
I went to /usr/local/lib/roundcubemail/ and started debugging the index.php. The long delay occurs when roundcube is calling the login_after-hook. Not many plugins register such a hook, so I checked all of them.
I disabled the carddav-Plugin by running a chmod 000 plugins/carddav (!!! write down the permissions beforehand so that you can restore them, ls -l plugins is your friend), and booom, login is fast again.
So: There seems to be a problem accessing the MIAB-Nextcloud instance. This seems to cause the timeout.