Would save space and loading time. Just as an idea…
So to dispel one notion here, it wouldn’t make any difference to loading time. Scrivener does not load everything inside of your project all at once, that would be extremely inefficient and essentially negate the primary advantage of using a folder & file mechanism. It just loads whatever it needs, in order to display what you had left open in the main editor(s) the last time you closed it, along with some infrastructural files.
Now speaking on space, what I am unclear on, with this request, is what advantage there would be to storing what will become an inert lump of inaccessible data inside of a project? All that will do is bloat the project size (even if by less than it was before), make backups larger (and take longer); there would be no material advantage to having this.
So, given that, why not just use File ▸ Save As..., to create a second copy of the project, giving it a name like “OriginalName - Part One.scriv”, and then in that second copy, delete everything except for Part One and whatever it would benefit from having around outside of the Draft folder. Empty the trash, and then close it, returning to your main project. Now you’ve got an archived copy of your work that has all of the original metadata, settings, internal links and other good stuff preserved within it, and it’s easily accessible if you need to reference something, but it’s no longer cluttering up the main project.
And sure, if you want to, you can also compress it rather than leaving it around as something you can open, using whatever algorithm you find most beneficial—but with it now out of the way and something you can load independently on the side, as needed, there would be much less reason for doing so I think.
Lastly, have a look at this post, on using multiple projects together. It might be useful, particularly if you would like to refer to elements of Part One from the other books, via inter-project linking.