This is a common misconception. Electron is a really tiny slice of these two text editors. Atom and VS Code are text editors and they have completely different code bases for the text editing, so there isn't anything that can be moved upstream.

Monaco, the text editor at the heart of VS Code, is just a lot better (and older and more mature) than Atom's. Perhaps Atom will eventually catch up or simply switch over to Monaco.

Yeah. I thought building a text editor in a web shell was incredibly stupid until I tried VSC. It's still more memory hungry than I'd like, but it's really fast—as fast as Vim for me (but noticeably slower than NeoVim).

Did MS actually modify Electron, did they just write better JS, or some of both?

The reply above yours seems to think it's "just" Electron with better JS, but I'm pretty sure Atom doesn't have DirectX acceleration ;)

It's open source, check by yourself:

https://github.com/Microsoft/vscode/blob/master/build/gulpfi...

which use

https://github.com/joaomoreno/gulp-atom-electron

to fetch electron