Something you can try (in this specific case something your girlfriend can try because she hosts the server):
You can set the amount of world data the server sends to the clients, measured in chunks in each direction of the player.
In the server folder is a file called server.properties with some keys to configure some basic server settings. On of these keys is called view-distance which is set to 10 as default. (Accepts an integer value between 3-15)
You can edit this file with a basic text editor (notepad for example). Try to lower the setting so the server sends less chunks to the clients. Don't forget to restart the server after changing server.properties.