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