Other idea: Something like Proof-of-Elapsed-Time.
A click on the verify button requests a token from the server. The server performs the action if the token is old enough.
-> No battery drain; equal waiting times for all users.
Does nothing to filter out bots. It just acts as a rate-limiter per IP address.
This is correct, but neither does the WeHateCaptcha approach. Thus spending time is better for the environment then spending energy ;)
No, it filters bots because spammers don't want to waste computing power/electricity. The delay is not the point.
Because a bot can request multiple tickets in a row, then submit them rapidly as they expire. Proof of work means the bot did the work for each request.
Why would that be a problem for an automated system?