I have a question, what if there are no hosted images, no hosted videos, no custom fields on server just a little text and links, would it be easier to handle that much posts?
Basicly yes. Of course. When you think about mysql, every meta data you need for a post is basicly a more complex querry, or think of it, like an extra request, to the db.
You mostly come to some douzends "requests" to the db anyway, because WP has to ask the DB for some shit like configs and stuff like this - it has to render every single page with the related informations in the DB.
So, if you reduce the requests, like by not using custom fields and stuff like this, your DB will perform better.
But thats a bit oversimplified as well - search querries and in particular random querries are very intensive again.
Your DB will be the very first bottle neck...
At 10 mio pages on a medium class server, this might work. I made a setup some years ago, stretching to 2mio Woocommerce Articles, wich have even more mess in their meta, with some DB tweaking, this can work. At least at medium load - a problem that might happen to you, is that the search engine bots will basicly ddos you.
What you can do as well, is just sourcing out your posts to backend server, that will serve you the content as json for example and then working with parameters, so that WP isn't even working, just requesting the article for example by the Get parameter given.
So you would be able, to integrate stuff into your existing WP.