Question: It slows down the browsing, the browser consumes 99% CPU for like 2 seconds after a page is loaded. And editing a page with some mathjax expressions is **hell slow**, probably because of preview. Why can't you do the same on server side, at least partially? There is a golden rule: don't do on client's side what you can do on server's side with the same result. And don't do something in javascript that you can do without.

Answer: 
That clause is against people inventing their own primitives, typically classical ciphers. New classical ciphers created by beginners aren't interesting, and evaluating a new modern primitive is far too much work for a question.

If you designed a proof of work system building on standard primitives that should be acceptable. We have plenty of nice questions where people designed their own higher level schemes. If the scope is too broad, it helps to ask specific questions about certain security properties of the scheme, instead of a full evaluation.
For the record, the FAQ doesn't say what you seem to think it says.

We're not interested in questions from an amateur who has invented a new homebrew encryption algorithm and challenges us to cryptanalyze it; those questions are rarely of interest to the Crypto.SE audience. However, if you have a question that's likely to be of broad interest to "software developers, mathematicians and others interested in cryptography", then give it a try.