This here is the implementation of sha256 in the slow language JavaScript:

const msgUint8 = new TextEncoder().encode(message);
const hashBuffer = await window.crypto.subtle.digest("SHA-256", msgUint8);
const hashHex = new Uint8Array(hashBuffer).toHex();

You imagined that JS had to have that done from scratch, with sticks and mud? Every OS has cryptographic facilities, and every major browser supplies an API to that.

As for using it to filter out bots, Anubis does in fact get it a bit wrong. You have to incur this cost at every webpage hit, not once a week. So you can’t just put Anubis in front of the site, you need to have the JS on every page, and if the challenge is not solved until the next hit, then you pop up the full page saying ‘nuh-uh’, and probably make the browser do a harder challenge and also check a bunch of heuristics like go-away does.