> Sure, you could crank up the difficulty until it makes the cost of the attack too high, but then all your users will get mad - that's basically what CloudFlare does and it attracts non-stop criticism.

This is interesting, where can I read more about both sides?

I do think you hit the nail on the head here, with resource symmetry being the center of the give-get here and making the botnet run warmer is the win (because it requires larger botnets, because it's more detectable, etc.). I think that this might make me upset as a client, but if it's economically advantageous for the server, then I'd still tolerate it to get the server's content.

I think that's why you need auth to be first-class in such a protocol. Yes, auth might be painful and annoying and require that your browser essentially mines hashes for a while on connection setup, but then after that presumably your website would be as fast as usual, and perhaps there might be a way to safely cache such authentication with a lease for valid users. Perhaps this kind of approach is too anti-internet, though, since it basically says that to do any useful backend work you need to be a registered user for some backend service.