Everyone saying "This isn't a speedup, this is VSCode stopping being dumb" hasn't read carefully enough:
The speedup is from taking extension code built on a limited API and implementing it inside VSCode, using more information while doing so.
I read it as extension developers will never be able to match core since their api is limited.
I mean, yes? That's kind of the point of extensions; to provide additional functionality through specific APIs.
Chrome/Safari/Firefox extensions certainly don't have full access to everything the browser can do. Nor can IntelliJ plugins. Nor can... practically any other implementation of extensions.
https://github.com/microsoft/vscode is available if a developer wanted to contribute something to core, anyway.