It's fast, and isn't entirely an enormous executable (40-ish MB).
However, most Electron apps are slow, laggy, and 100+ MB.
It seems great when used by an expert, or great for a stopgap when you need to get something into public hands, but you plan on replacing it with something smaller and faster.
Yes, because it's common in (Node)JS land to add many dependencies. I guess MS simply coded most of the stuff on their own, which keeps the size low.
Also, they use TypeScript and I heard that it reduces function polymorphism, which can increase performance (but I don't know how much). Also, as far as I know, VSCode runs plugins in different processes, which makes the main process snappier.
But I heard that the static typing (implicitly) leads devs to write more monomorphic functions, which seem to be much easier to optimize by the JavaScript-VMs. But yes, you can write such code in JavaScript yourself.
The thing is that writing polymorphic stuff is a bit more effort in TypeScript, if you don't want to throw 'any' around everywhere.
VS Code does, its fast, extensible and has been the launching pad for some very cool things like a unified syntax protocol.
Atom does, but quickly becomes unuseable with too many extensions, and so I find myself not using it. What did the extra bloat give it over something like Sublime?