Refactor _json to use the cache rather to reconstruct the whole form on every ajax request.

Well reasons should be more then clear, this costs a lot of performance, rebuild the form, call all alter methods and stuff on every ajax call.

And all this to get one element of those thousands. Fixing this will impact HS performace a lot, i guess.

Fileframework/attach module shows how it should be done. It does exactly the same, even with file uploads, but actually not rebuilding the whole form