I’d say that a site at that scale is not a goal. For the current iteration of Sparkle anyway. On the other hand a single person can do more with Sparkle in a day than a small team can do in a week, depending on the site kind and content.
Again scaling is of interest, but not necessarily something to be solved without changing how Sparkle works.
@ mOehlschlager Keep in mind that if you split a site into subprojects, you loose some capabilities, e.g. search.
Very bad thing this…
I see it that way.
If your project grows so big that you need to think you need to split it into different projects then I believe Sparkle is not the right tool for you to stay productive.
In that case you really need to look into a content management system, those systems are made for a large number of pages.
By saying CMS I am not referring to WordPress, that would be the worst choice.
If you really want to stick with Sparkle then I would suggest getting one of those CMS systems that you can integrate into a static website via embed Html. There are a couple of those CMS out there but not for free I am afraid.
I am not aiming to create such big websites, that’s why Sparkle is just perfect for me and satisfied how the page management, of course it needs a bit of improvement but that will come over the next versions.
I believe it needs to be addressed different things in Sparkle before the development team things on how to manage megaproject.
that is my five cents. 
@MiWe You make good points.
I’ll offer another point of view. Sparkle grows with you. When I started using it, it didn’t even support having folders and subfolders. Our blog was manually created, etc. All that has been addressed and then some.
Of course, if there are features that you need now, then you need another solution.
I’ve often wondered if Sparkle could make fuller use of Sections. These are (or could be) sub-projects, especially if they were folders in the published site (creating index.html files, as would separate projects). Perhaps an option “Publish Section as folder” would make this possible.
Without knowing Sparkle’s coding structure this may be impossible but if these sections could be loaded for editing separately (a sort of hyperlink panel) loading speed might improve.
Appreciate the spirit and the suggestion, but within Sparkle’s internal structure this wouldn’t be a shortcut to faster loading, and from a UI point of view I’m not sold that the sections-as-projects UI is beneficial when thinking about the restrictions they would impose.