It doesn't need to be anywhere near as resource intensive as a crypto currency since it isn't used for security. The goal is not to stop bots altogether. The goal is to slow down the crawlers enough so that the server hosting the service doesn't get pegged. The bots went from being respectful of server operators to hitting pages millions of times a second. This is made much worse by the fact that git hosting services like Forgejo have many links many of which trigger the server to do computations. The idea behind Arubis is that a user really only has to do the PoW once since they aren't browsing to millions of pages. On a crawler it will try to do tons of proofs of work which will bog down the crawling rate. PoW also has the advantage of requiring the server to hold minimal state. If you try to enforce a time delay that means that the server has to track all of that.

It is also important to realize that Anubis is a act of desperation. Many projects do not want to implement it but they had no choice since their servers were getting wrecked by bots. The only other option would be Cloudflare which is much worse.