If Google really wanted to make the web fast, they would add native support for text/markdown mimetype in Chrome and ask publishers to just create .md files instead.
text/markdown is a bad idea because it's not backwards compatible. It should be text/plain; markup=markdown, since Markdown is perfectly readable without any software support.
backwards compatible with what? In my experience anything served as `text/*` that isn’t more specifically handled (eg `text/html`) will be rendered as text/plain because the type is still `text`.
See https://blog.freron.com/2011/thoughts-on-writing-emails-usin... for more thoughts on this topic.