What other problems are you having? Without feedback we can’t improve things ;-)
the efficiency of an editor with markdown card containing for example 50k characters should be improved
What problems are you experiencing with a large markdown card?
Straight markdown->mobiledoc is unlikely but we are working on html->mobiledoc and looking at ways to make that easier via tools/libraries and possibly with the v2 Admin API. Seeing as markdown->html is what markdown is designed for then there’s not much of a jump to markdown->html->mobiledoc, in fact if you paste markdown directly into the editor you’ll see that the conversion is already handled for you.
There are caveats however, mobiledoc is a little more restrictive in certain areas than markdown which allows mixing in free-form HTML. As such it’s possible that free-form HTML or scripts that are included in the markdown may be lost in the conversion.