WordPress Permalink Performance

Hello, I am trying to figure out some stuff related to permalink performance. Currently, my permalink structure is /%postname%/. I was going to need to add potentially thousands of pages to wordpress but after doing research, I found out that wordpress really isn’t suited for this due to the way permalinks are handled (because of the time spent deciphering between a page and a post). Well, one thing I don’t understand then is how having a lot of posts is different than a lot of pages. If I make all 10,000 of my pages posts instead of pages then for each post a user goes to, it should still have to match between 10,000 posts making it really slow I would imagine. And thus, the only time this is really an issue is when you look at something like a category page. You would want it to not have to look through the pages or posts first or else it could slow down that lookup. But the funny thing is, when you land on a single post, it still would have to go through all the post matches. So how is it that this really slows it down any more than having a whole bunch of posts? Help me understand, thank you.