Yes — that’s exactly the problem. Handling ten thousand concurrent users with CGI is easy, even on 2000-era hardware. Ten thousand concurrent users might be four requests per second. But ten thousand concurrent users using websocketd means you have ten thousand processes. And if you’re doing some kind of pub/sub thing, every once in a while, all of those processes will go into the run queue at once, because there’s a message for all of them. Have you ever administered a server with a load average over 1000?

Still, the O(N) scheduler work in current Linux might make that kind of thing survivable.