在vue2项目配置一个打包速度很快的webpack文件

要在Vue2项目中配置一个打包速度很快的Webpack文件，可以参考以下步骤：

1. 使用`vue-cli`创建一个Vue2项目：

   vue create my-project

2. 安装`webpack-merge`插件，用于合并Webpack配置：

   npm install --save-dev webpack-merge

3. 在项目根目录下创建一个`build`目录，并在其中创建一个名为`webpack.common.js`的文件，用于存放公共的Webpack配置：

   const path = require('path');
   const webpack = require('webpack');
   const { VueLoaderPlugin } = require('vue-loader');
   
   module.exports = {
     entry: {
       app: './src/main.js'
     },
     output: {
       path: path.resolve(__dirname, '../dist'),
       filename: '[name].js'
     },
     resolve: {
       extensions: ['.js', '.vue', '.json'],
       alias: {
         '@': path.resolve(__dirname, '../src')
       }
     },
     module: {
       rules: [
         {
           test: /\.vue$/,
           loader: 'vue-loader'
         },
         {
           test: /\.js$/,
           loader: 'babel-loader',
           exclude: /node_modules/
         },
         {
           test: /\.(png|jpe?g|gif|svg)(\?.*)?$/,
           loader: 'url-loader',
           options: {
             limit: 10000,
             name: 'img/[name].[hash:7].[ext]'
           }
         },
         {
           test: /\.(woff2?|eot|ttf|otf)(\?.*)?$/,
           loader: 'url-loader',
           options: {
             limit: 10000,
             name: 'fonts/[name].[hash:7].[ext]'
           }
         }
       ]
     },
     plugins: [
       new VueLoaderPlugin(),
       new webpack.optimize.ModuleConcatenationPlugin()
     ]
   };

4. 在`build`目录下创建一个名为`webpack.dev.js`的文件，用于存放开发环境的Webpack配置：

   const merge = require('webpack-merge');
   const common = require('./webpack.common.js');
   
   module.exports = merge(common, {
     mode: 'development',
     devtool: 'cheap-module-eval-source-map',
     devServer: {
       contentBase: '../dist',
       hot: true
     },
     module: {
       rules: [
         {
           test: /\.css$/,
           use: [
             'style-loader',
             'css-loader'
           ]
         },
         {
           test: /\.scss$/,
           use: [
             'style-loader',
             'css-loader',
             'sass-loader'
           ]
         }
       ]
     },
     plugins: [
       new webpack.HotModuleReplacementPlugin()
     ]
   });

5. 在`build`目录下创建一个名为`webpack.prod.js`的文件，用于存放生产环境的Webpack配置：

   const merge = require('webpack-merge');
   const common = require('./webpack.common.js');
   const { CleanWebpackPlugin } = require('clean-webpack-plugin');
   const MiniCssExtractPlugin = require('mini-css-extract-plugin');
   const OptimizeCSSAssetsPlugin = require('optimize-css-assets-webpack-plugin');
   
   module.exports = merge(common, {
     mode: 'production',
     devtool: 'source-map',
     optimization: {
       minimizer: [
         new OptimizeCSSAssetsPlugin()
       ]
     },
     module: {
       rules: [
         {
           test: /\.css$/,
           use: [
             MiniCssExtractPlugin.loader,
             'css-loader'
           ]
         },
         {
           test: /\.scss$/,
           use: [
             MiniCssExtractPlugin.loader,
             'css-loader',
             'sass-loader'
           ]
         }
       ]
     },
     plugins: [
       new CleanWebpackPlugin(),
       new MiniCssExtractPlugin({
         filename: 'css/[name].[hash].css'
       })
     ]
   });

6. 修改`package.json`文件，添加两个命令用于启动开发和生产环境的Webpack：

   "scripts": {
     "serve": "webpack-dev-server --config build/webpack.dev.js",
     "build": "webpack --config build/webpack.prod.js"
   }

7. 运行以下命令启动开发环境的Webpack：

   npm run serve

运行以下命令构建生产环境的Webpack：

   npm run build

这样，你就可以在Vue2项目中配置一个打包速度很快的Webpack文件了。其中，我们使用了`webpack-merge`插件来合并公共配置和环境特定的配置，使用了`clean-webpack-plugin`插件来清空打包目录，在生产环境中使用了`mini-css-extract-plugin`插件来提取CSS，使用了`optimize-css-assets-webpack-plugin`插件来优化CSS。