VSC dev here. Correct, perf is a priority for us and it's no black magic. A simple one is to keep your bundles small. VSC's main bundle is less than 9MB, Cloud UI seems to be 15.
Which is a problem with the dev environment. If it takes a lot of effort to make an application performant then most of the applications written in that environment will be slow.
It isn't so much about environment as it is experience. It is incredibly easy to bring in libraries and components that bring the entire jungle for want of a banana. Especially graphing/charting libraries. It's also about focus, the focus is on the feature not the performance/feel.
It's not a priority with many of Google's current apps.