The cause lies in the configuration of your browser settings. By default, this allows 2 or 6 parallel downloads at the same time. To change the setting, open e.g. Firefox the following page: about:config
Then search for the setting network.http.max-persistent-connections-per-server.
Here you can set the value from 2 or 6 to 60 or 100 parallel downloads. A restart of the browser is usually not necessary.
More information can be found in our manual: