I ever tried different solutions/plugins. Finally, I settle with a solution by combining 2 plugins to write/edit posts in Markdown in WordPress:
The rendering on the fly uses too much CPU cycles? This is a cost. But I always use WP Super Cache on WordPress.
for Markdown editing. It is a beautiful and easy to use Markdown editor for WordPress with a set of Markdown buttons. The basic function is editing. Saving the Markdown code of the post in WordPress and use php Markdown to render it when serving the post. Additionally, it can render the Markdown to HTML and HTML to Markdown which, I find, is very useful for editing old posts written in HTML. I like the undo and redo feature much too---we always make mistakes. Especially when rendering HTML to Markdown, we may mess old posts up and the undo feature will save us.