webpack rebuild 速度太慢
工程的开发环境使用`express`+`webpack-dev-middleware`+`webpack-hot-middleware`，热加载+网页的错误提示开发起来很爽。
之前webpack每次启动的build速度都比较慢，后来参照这篇文章做了优化：
开发工具心得：如何 10 倍提高你的 Webpack 构建效率
加上dll plugin和happypack等之后，构建速度一下子由120s缩短到30s以内，随着工程越来越大（12万业务代码），rebuild速度越来越慢，现在一次rebuild速度大概得12s左右，实在是不能忍受。
哪位有这方面优化经验的，还请不吝赐教。
There are quite a few conditions that can contribute to slowness. From personal experience, I have found that the best way about trying to solve it, is to Measure, Profile, and Diagnose the issue first. 
When you run webpack, instead run it with node and the following flags: 
`node --inspect --debug-brk ./node_modules/webpack/bin/webpack.js`
Then open up Chrome Browser and goto `chrome://inspect` and click 'Open Dedicated Node Debugger'. 
This will give you debug, breakpoint, and profiling features that you can use to measure and analyze what functions and areas of webpack are taking the longest. Once you have found that out, you can start to determine exactly what is causing the issue. Here is a great post (although a little outdated for the instructions), that shows how these steps might look like
If this doesn't help, then you can try other webpack build features like DllPlugin, using `devtool: 'eval-source-map'`, look for specific loaders that have caching features, as well as our new cache-loader.