So your solution is to technically waste electricity to replace captcha? It's for sure an interesting concept, the first point and low-end devices requiring 20+ seconds to pass are not a very good points to sell your service.
20 seconds would be a fairly high difficulty. It's up to the site owner to decide what makes sense for them.
If anybody comes up with a useful computational task with a small bundle size that can be verified cheaply that would be the holy grail - until then the computation is only there as a form of hashcash.
It’s also nonsense to compare the computational cost of N seconds of sustained, maxed out useless computation to the milliseconds of compute time needed to decode an image, or the minimal power usage of waiting on network data.