I was wondering the very same question, thanks for bringing it up. I’ve also noticed that some Kirby calls are way more expensive than others, but with the right optimization a site of this size can definitely run smoothly.
For example, there’s this case here on the forum: 9000 articles website crashes. The site was actually based on my Tablog theme. The main issue there was simply that I hadn’t included pagination (my oversight—I didn’t expect someone to run 9000 articles on a blog 
). Once pagination was added, the site has been running fine.
That said, I do have a related question for @aengustukel and @bnomei (or anyone with real-world experience at this scale):
How do you usually handle storing orders? Do you rely on an external service like Snipcart, Stripe Checkout, etc.? Or do you keep them as regular Kirby pages?
I’m curious if you see potential bottlenecks or pitfalls with storing thousands of orders as content files, especially in an e-commerce setup like the one described here (4k+ products, user accounts, etc.).