At the time of writing, this page is 44,829 bytes in size and though that may not sound like a whole lot, it has become painfully slow to edit. Maybe move the section for archived extensions to a subpage and transclude that here?
Topic on Talk:Extension default namespaces
With or without VisualEditor? I do agree that it's a long page, but in regular edit mode (without VE), it loads just fine for me on a 10+ years old laptop. Some pages just end up long because of the way it simply is, and this page is a prime example of that.
I was actually editing the page without Visual Editor. Of course, there is a real possibility that it's just my browser, although I don't experience anything of the kind while writing the present post.