In August 2019 SharePoint Framework 1.9.x was released. Among different changes also support for Webpack 4 was introduced. What does it mean for us? It means slightly improved build speed, support for a wide range of plugins and better tree-shaking. 

What is webpack tree-shaking exactly? In simple words, webpack is smart enough to automatically remove "dead modules" (in other words unused code/modules) from your resulting bundle. It reduces the size of the resulting bundle, thus improves load performance. More...