Also some cool demos on his website: http://madebyevan.com/

written in his own language ...

The title doesn't mention JavaScript nor is the editor written in or exclusively compiled to it. Further, modern JavaScript engines are very fast. The main performance bottleneck in web-based editors like Atom or Light Table is the DOM. The web target of this editor only makes use of two DOM nodes - the <canvas> that everything renders to and a hidden <input> for capturing user input.

ha, I did try it with ~1.2e6 lines of unminified JS and it kept its cool, very smooth scrolling and multiline editing.