Update! I had a look at this and I must admit a change like this would mean a lot more work than I initially would have thought. If the sitemaps were static it wouldn’t probably be any problem, but since they’re dynamic it comes with lots of complex problems involving redirect rules (and what I think extra database calls) etc… And of course the whole plugin would also have to be pretty much rewritten (it’s too “hardcoded” sigh).
So just so no one gets their hopes up I’m saying that an update like this won’t happen at the moment. Perhaps in the future though, it’s still a necessary part for making the plugin to also work for sites with huge amounts of posts.
But it was initially intended for smaller sites, and I’ll guess that’s how it’s gonna stay for now. There’s lots of other sitemap plugins out there that’s probably more fitting for massive ones. And I would recommend trying one of those if that’s the case with your site.
Anyway if the bomb would drop and work to support massive sites should begin. I will announce it here of course!