Marvin is doing wonderful work in the JS ecosystem around performance. It has been largely focused on tools and node, however, he did have an interesting set of things to say about how they optimized performance in Preact as well on his website that was really interesting too.

One thing I've noticed is the rampant duplication of polyfills and babel helpers. To the point that I now have overrides setup via pnpm and I re-write many imports of polyfills to point at my own shims, which simply re-export existing functionality native to the language, most of the time.

For smaller utility packages, I often simply clone the repo and copy things over that we need, or copy the src right out of the node_modules folder if possible, then I strip away all the superfluous imports (and often convert from commonjs to ESM if needed)

Saves so much headache, its better for users, smaller builds etc.