10-15 seconds is adjustable. This is default settings. You can make it smaller or bigger depending on amount of spam or attacks your are getting. Setting it to very small value will still deter many bots who do not run Javascript at all.
Now regarding waiting. Just registering on this forum to answer in this topic I had to answer question, enter reCaptcha and wait for validation mail. both took definitely more than 15 seconds. I would say it make sense to set complexity (i.e. amount of work necessary) dynamically based on what form is that (i.e. make it harder on registration form,) what is user level on forum is (if they have many posts - make complexity minimum or remove it altogether,) etc.
I just started this project and gathering feedback. I am already implementing some features people asked for, so feel free to ask questions please
As for bitcoin mining - you are very close. It is actually dogecoin proof-of-work. Basically idea is that each interaction you do with website cost you something. Once I understand that each interaction I do comes with cost - I would think twice before posting junk. And obviously it makes job of bots even more expensive, since they need to post tons of spam to get their rankings/leads/subscriptions.
Anyway, if someone have questions - I am always available at pavelkaroukin@gmail.com
