yes, there are many points that I can state
- WP does not load all the posts but only the first page
- if you query for a category you will need more time, if there are many posts
- if you use a VPS server you can manage more posts comparing with the Share Hosting
- if you use load ballance is better
- if you insert,delete, update posts THEN if you tune the database the site is going to be faster, so a lot of posts means slow database
- But even the sites with a lot of posts can be tuned and run fast
