Add caching rule to speed up subsequent imagemin builds.

generator/src/develop.js

@@ -111,6 +111,7 @@ function webpackOptions(production, routes, { debug, manifestConfig }) {
       ]),
       new ImageminPlugin({
         test: /\.(jpe?g|png|gif|svg)$/i,
+        cacheFolder: path.resolve(process.cwd(), "./cache"),
         disable: !production,
         pngquant: {
           quality: "50-70",