[saganus]

Maybe there is a way for the server to ask the client to do the work?

Something similar to proof-of-work but on a much smaller scale than Bitcoin.

[mrheosuper]

just add some delay to your response, we don't have to waste any more energy on meaningless calculation.

[01HNNWZ0MV43FF]

Adding delay means you have to keep more connections open at a single time. Parallelism doesn't favor a server if your problem is already a small server getting hit by a big scraper

[Ma8ee]

How expensive is it to just keep a connection open?

[swiftcoder]

About 20 kilobytes of socket + TLS state, if you've really optimised it down to the minimum. Most server software isn't that lean, of course, so pick a framework designed for running a million or so concurrent connections on a single server (i.e. something like Nginx)