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You're mixing the two cases. If every page is limited then yes they have to work hard, but still get delay/machines visits per second. But a human will have to wait the full delay every time a page is visited. This is unacceptable for modern browsing.

If the delay is only once per, let's say a domain, then you don't do anything against the spammers, they only have to wait a full delay once.

Because that wouldn't stop them. Let's rather say instead everyone gets a fixed delay in seconds. Then the spammers will just wait out that delay and then spam. Even if the delay is on every single page visit, that doesn't harm a botnet, because they can still do delay/machine_count visits per second.