@Bmosbacker, your homework assignment is … Choose your favorite language platform and begin to write your own plain text and Markdown editor. Lots of parsing and display hacks required.
Or read @snelly 's exploration and ultimate abandonment of writing his own plain text editor. Thinking about building a Markdown editor for macOS and iOS - Software - MPU Talk
I’ve opened large files in iA Writer so, in that sense, it works. But the performance is so bad that I would not want to use it.