Per https://webpack.js.org/blog/2020-10-10-webpack-5-release/#ge... :

- Improve build performance with Persistent Caching.

- Improve Long Term Caching with better algorithms and defaults.

- Improve bundle size with better Tree Shaking and Code Generation.

- Improve compatibility with the web platform.

- Clean up internal structures that were left in a weird state while implementing features in v4 without introducing any breaking changes.

- Prepare for future features by introducing breaking changes now, allowing us to stay on v5 for as long as possible