VS Code doesn't provide direct DOM access to extensions authors, that's definitely a deciding factor.

But a couple of years ago even simply writing inside Atom without using any extension felt sluggish to me.

TypeScript gets downcompiled to JavaScript anyway, so I don't think that's directly a contributing factor, but maybe because of the type system they can spend less time debugging and more time optimizing.

It's because much of the guts of VSCode is written in C++ and optimized.

Because, you know, Microsoft.

No it's not.

The file searching code is in Rust (IIRC) and runs as an external process, but that is about it.

So eletrons lets you code part of it in C++?

You can have native modules in Node.js, and you can use Node.js inside Electron.