Important !!!
If you test your game in studios your client is the server but if a random player wants to join the game they have to load the whole map at ones which could take a lot of time
and you would get less players or no players if your map is that big here is what you can do:
to prevent the player from falling trough the ground you have to change the streaming mode of the parts which arent loaded fast enough(falling through the ground) to:
persistant = keeps the model loaded forever
I would realy recoment it because some players on mobile could have big problems