When working with large Sass frameworks such as Bootstrap, it's easy to simply add them to the top of your /src/styles.scss file to get started. However, to get the best potential build performance, it's better to import your vendor styles through a separate vendor.scss file. This allows the Angular-CLI build to limit the regeneration of its stylesheets when you rebuild to only what has changed. This is because of a tree-shaking optimization built into Angular-CLI's WebPack build system that doesn't recompute unchanged branches of content. By separating your library styles such as Bootstrap from your own custom Sass styles, Angular-CLI will only recompile the part that has changed. Simply move all your bootstrap imports and variable customization over to this new vendor file and add it above styles.scss in your .angular-cli.json configuration:

"styles": [
  "vendor.scss",
  "styles.scss"
],