Interesting, subtle difference but I always thought of captchas as having computational difficulty, but that's clearly not the point as you say. The cost is not

Interesting, subtle difference but I always thought of captchas as having computational difficulty, but that's clearly not the point as you say. The cost is not compute but developer time. If you manage crack it at 1mhz per captcha or 1ghz or 1000ghz, it makes no difference, as the bottleneck is the network identifier (ip address/block) While still a type of PoW, these economics are different than offline mechanisms like password hashing or crypto. Where a 1ghz cost is still significantly different than 1mhz.