perf: revert babel/ts thread behavior

packages/@vue/cli-plugin-babel/README.md

@@ -22,6 +22,10 @@ module.exports = {
 
 [cache-loader](https://github.com/webpack-contrib/cache-loader) is enabled by default and cache is stored in `<projectRoot>/node_modules/.cache/babel-loader`.
 
+## Parallelization
+
+[thread-loader](https://github.com/webpack-contrib/thread-loader) is enabled by default when the machine has more than 1 CPU cores. This can be turned off by setting `parallel: false` in `vue.config.js`.
+
 ## Installing in an Already Created Project
 
 ``` sh

packages/@vue/cli-plugin-babel/index.js

@@ -1,5 +1,6 @@
 module.exports = (api, options) => {
   const { genCacheConfig } = require('@vue/cli-shared-utils')
+  const useThreads = process.env.NODE_ENV === 'production' && options.parallel
   const cliServicePath = require('path').dirname(require.resolve('@vue/cli-service'))
 
   api.chainWebpack(webpackConfig => {
@@ -29,6 +30,12 @@ module.exports = (api, options) => {
           .options(genCacheConfig(api, options, 'babel-loader', 'babel.config.js'))
           .end()
 
+    if (useThreads) {
+      jsRule
+        .use('thread-loader')
+          .loader('thread-loader')
+    }
+
     jsRule
       .use('babel-loader')
         .loader('babel-loader')

packages/@vue/cli-plugin-typescript/README.md

@@ -18,6 +18,10 @@ If opted to use [TSLint](https://palantir.github.io/tslint/) during project crea
 
 [cache-loader](https://github.com/webpack-contrib/cache-loader) is enabled by default and cache is stored in `<projectRoot>/node_modules/.cache/ts-loader`.
 
+## Parallelization
+
+[thread-loader](https://github.com/webpack-contrib/thread-loader) is enabled by default when the machine has more than 1 CPU cores. This can be turned off by setting `parallel: false` in `vue.config.js`.
+
 ## Installing in an Already Created Project
 
 ``` sh

packages/@vue/cli-plugin-typescript/index.js

@@ -26,6 +26,12 @@ module.exports = (api, options) => {
       options: genCacheConfig(api, options, 'ts-loader', 'tsconfig.json')
     })
 
+    if (useThreads) {
+      addLoader({
+        loader: 'thread-loader'
+      })
+    }
+
     if (api.hasPlugin('babel')) {
       addLoader({
         loader: 'babel-loader'