This is an old idea known as hashcash. https://en.wikipedia.org/wiki/Hashcash https://en.wikipedia.org/wiki/Hashcash Newer variations (such as argon2) are tuna

This is an old idea known as hashcash. https://en.wikipedia.org/wiki/Hashcash https://en.wikipedia.org/wiki/Hashcash Newer variations (such as argon2) are tunable so you can include memory footprint and cpu-parallelism. There also are time-lock puzzles or verifiable delay functions that negate any parallelism because there's a single answer which can't be arrived at sooner by throwing more cores at the problem.