This comment:
> The closest to any discussion of these numbers that I saw was a mention that it may take up to 20s on mobile; for comparison, the much-hated image CAPTCHA takes about 6-12s on average for native English speakers, and 7-14s for non-native speakers.
Missed this quote from the website:
> As soon as the user starts filling the form it starts getting solved
> By the time the user is ready to submit, the puzzle is probably already solved.
The time spent solving reCAPTCHA is active user involvement. The time being spent on Friendly Captcha is passive and can overlap with time being spent filling out a form.
"up to 20 seconds" was also seemingly presented as a worst-case scenario. Most users' devices would presumably be faster than that, but I don't know how the author researched that conclusion on how performance scales. Friendly Captcha does report back some information on how long it is taking users to solve the captcha, and it looks like website owners could use that to adjust the difficulty based on the needs of their specific audience and how tolerant they are of untargeted spam.
The stuff you point out about Blake2b seems entirely legitimate, and I wonder if an Argon variant would be more appropriate to avoid specialized hardware being quite so problematic.
Personally, I really like the idea of Friendly Captcha. Certainly, there are problems with any captcha implementation. People can rant for many, many paragraphs about websites that use reCAPTCHA... I'm not surprised to see someone ripping apart a different captcha system. The ideal solution would be for spammers to just stop being so obnoxious... but good luck with that plan.