I'm creating a template with many parameters available in its backend. Basically I have about 8 blocks in the template structure (index.php) and each block have about 30 options (background, content type, layout, animation etc) so I have about 300 options totally in the template backend. This makes the template backend is loaded very slow when opened or saved. Is there any native way (I didn't use any template framework) to make all of these 30 options are loaded in the separated / child xml so it will be called by demand and the template backend loaded faster?
Because you did not use any known template frameworks, I'd be very very surprised if anyone will be able to give you a direct answer to your question. I googled a few, and found Using a Joomla Template Framework to Design your Site, which outlines the pros/cons of the popular frameworks that are out there.
Again, not knowing how your code works at all, there's no way we can guess at a native way to implement parameters, but if you look at these known and accepted frameworks, perhaps you'll see a way that it's done with them and you can incorporate it into your framework.
There is no native way to load separated / child XMLs for template configuration. In fact, any XML loader checks if it is parsing a valid XML. If your configuration is separated in several files, it is not a valid XML until you merge all of them.
I think your issue is not in the XML size. The size of the configuration Html form may be the cause of the slow load. Most template networks simplify the configuration with several smaller forms or wizards.
It's now possible with the Sub Form field function introduced in Joomla 3.6