title: Flow takes very long to start up because it checks node_modules
body: <p>I added <a href="https://flowtype.org/" rel="nofollow noreferrer">Flow</a> to my React/Redux/Webpack project and initially, it was great, I loved the type annotations. Over time, I noticed that the start up time of the project became really slow - around 120s; it used to be around 20s. Upon investigation, I realized that it was because Flow was scanning through all the JS files in <code>node_modules</code>.</p>
  
  <p>I attempted to ignore <code>node_modules</code> in <code>.flowconfig</code> by adding: </p>
  
  <pre><code>[ignore]
  .*node_modules/.*
  </code></pre>
  
  <p>The start up became fast again but Flow would complain <code>Required module not found</code> in places where I import external libraries from my code. </p>
  
  <p>A workaround suggested in this <a href="https://github.com/facebook/flow/issues/869" rel="nofollow noreferrer">Github issue</a> was to flowignore the <code>node_modules</code> and manually add interfaces for external libraries. This seems to work but is a hassle to maintain when new libraries are added into the project.</p>
  
  <p>It's frustrating to have to wait almost 2 minutes each time I start the project, are there any better ideas?</p>