The treeshakeability of three.js is not ideal at the moment (more information about this topic right here: https://github.com/mrdoob/three.js/pull/14654)
I’m currently working on the first step in order to improve this situation by removing all IIFEs from the codebase and then move towards ES6 classes. When I remember correctly, there is also an open issue in context of three.js's shader library that impedes tree shaking. If those issues are solved, you can expect smaller bundle sizes and a faster startup.