Comment by hombre_fatal

Websites aren't really fungible like that, and where they are (like general search indexing for example), that's usually the least hostile sort of automated traffic. But if that's all you care about, I'll cede the point.

Usually if you're going to go through the trouble of integrating a captcha, you want to protect against targeted attacks like a forum spammer where you don't want to let the abusive requests through at all, not just let it through after 5000ms.