Why not get the browser to do 10 seconds of cryptographic work, as per the original aim of Hashcash [0] (which was repurposed for bitcoin mining). The user cou

Why not get the browser to do 10 seconds of cryptographic work, as per the original aim of Hashcash [0] (which was repurposed for bitcoin mining). The user could be presented with a message letting them know what's happening and the ability to bypass it. It would not continue to use cpu once the sufficient work has been done to access the site. You wouldn't use the original algorithm, because a bad actor could use an old bitcoin miner to quickly finish the work. [0] https://en.wikipedia.org/wiki/Hashcash https://en.wikipedia.org/wiki/Hashcash