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.