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Waste their time with a very slow server. Like OpenBSD’s spamd. Think one letter per second for text based protocols.

Precisely that

Something like this https://github.com/die-net/http-tarpit

To delay incoming connections

https://en.wikipedia.org/wiki/Tarpit_(networking)

Cloudflare's specific methods are described in the article, it's likely some web crypto stuff or an extreme response that takes a long time to parse or handle.