Well, ST3 does 70% of what VSCode does, at 1/10th the memory and much faster, and it's written by a single person.

Besides, they're still writing in a typed "language" (ts), so it's not like the dynamic language nature of JS that allows the VSCode base code to be developed incredibly quickly.

What really allows them to develop it incredibly quickly is the plugin API -- which allows tons of npm stuff to be ported and tons of JS programmers to create plugins for VS Code. A native statically typed based core would allow for that too. In fact the VSCode team already have moved some VSCode processing to C++ libs exactly for speed (and of course node itself is C++).

Using DOM for the drawing is faster (for a convenience perspective, not execution speed) yes, but with MS resources they could have made a streamlined equivalent that's much faster, more memory efficient, and doesn't have all the baggage of a web rendering engine. In fact, they have several native alternatives lying around they could make cross platform.

As it is, VSCode will ever have the burden of rendering to the DOM.