[laughinghan]

The time being spent on Friendly Captcha is passive and can overlap with time being spent filling out a form.

Great point!

I wonder if an Argon variant would be more appropriate

The creators of Argon2 actually also created a memory-hard proof-of-work function they call MTP (for "Merkle Tree Proof", which is a terrible name, totally un-Googleable; I always have to search for the title of their paper, "Egalitarian Computing"): https://arxiv.org/pdf/1606.03588.pdf

A bug bounty for it was sponsored by Zcoin, which is nice. Zcoin is actually considering moving away from it, but mainly because the proof size of 200kb is prohibitive, which is less of a concern for a captcha system: https://forum.zcoin.io/t/should-we-change-pow-algorithm/477

I'm not surprised to see someone ripping apart a different captcha system

I really don't mean to rip it apart. I just wanted to see some discussion, any discussion, of the well-known flaws with the idea and what ideas OP has to address them.