VS Code is a perfectly fine and pretty performant app.
It's also had a ton of work done to make it performant. (Microsoft level resources)
Also, first time I'm hearing about Monaco. I thought VSCode was an Atom fork.
The text editor was based on Monaco, a Microsoft product. VS Code only borrowed Atom's multi-platform bundling technology (it was called Atom Shell back then, then renamed to Electron).
There's only 0.7% Rust code and not even any C++. And almost all of that Rust code is for the CLI, not the editor.
The rest is just plain TypeScript and other web technologies.
Also the performance improvements on the bultin terminal thanks to using WebGL as if it was a native app.