Parent portals or child portals are not any different, they are all sub-portals in the same DNN website install.  It's more of a resource (memory) issue than it is a performance issue.

No, there is not a built-in way to export a single portal and re-import it into another DNN website.  Although I believe some third-party modules do this.

Based on what you have said so far, I would recommend as I did earlier.
You want to be thinking of how you can build your application as a global application to create the groups within the same DNN website.
As you grow, you can then create the same global application on another server.
At that point you may want to have them run their own groups independently,or you may need for them to remain tied together under one umbrella for back-end processing
If you need all the servers to act as a single global application then you can use server load-balancing.

You are not going to be able to do this without customizing DNN with your own modules or some other third party modules.
It would be nice if you could install DNN and have it be the next MySpace, but you are setting your expectations too high if you think that can happen.
DNN is a web application framework.  It provides the foundation for you to build apps like MySpace, but it is not a vertical application with specific business processes.

If you are not comfortable with extending the framework to make it do these things, and you don't want to get someone else to do it, then I would recommend creating a process where you can easily deploy a brand new DNN install for every group.  Of course if you need them to be sharing some functions then you may be back to the one application. I'm not sure what your plans are, but this is why I said there are many things to consider.

Think of DNN as a pre-fabbed brick and mortar building that gives you all your plumbing, electricity, and even gives you some basic tools to help you keep track of customers.
Now you need to extend the framework with modules to add your display cases, and help you with the back-end business management like inventory control, shipping, etc.
Do you need an espresso bar for your new business? Maybe you need something that facilitates communication between your customers? That is where modules come in.