I see your concern is focused on polling case (e.g a chat room). As goes with the McDonalds example, a single cashier can reply the question "is my order ready?

I see your concern is focused on polling case (e.g a chat room). As goes with the McDonalds example, a single cashier can reply the question "is my order ready?" rapidly. While it takes seconds in real world, a client polling request would take milliseconds to complete, so it can still serve hundreds of clients in a given second. If you'd need more for that part, there can be more cashiers/apps to make "asking" part scale indefinitely (even you'd have 1 worker only).