title: Does Jest import every test file individually, or does it mount all the references one time before executing?
body: <p>I have several unit tests, and each module imports a lot of data resources from a JSON file via a method. This IIFE method is imported by every test module, and I'm trying to figure out why my tests are so slow. The JSON data is massive, so I'm guessing that the reason this is the case, is because every test imports the huge data as it is ran. If this is the case, I'll have to modify the method, to return only specific data sets.</p>
  
  <p>So my question is, is the data imported every single time each test module runs, or only once when I execute <code>npm run test</code>?</p>
  
  <p>A basic example of the structure I have: 
  <a href="https://codesandbox.io/s/beautiful-frost-nl9vu?file=/src/tests/data.json" rel="noreferrer">Codesandbox</a>. Say <code>test1.js</code> requires only <code>dataset_1</code> key from the data json, and the other two test files need <code>dataset_2</code>. Would it be more performant to write a method that returns the required data props to the test file that it is invoked in, or it doesn't matter? Trying my best to phrase this question correctly, please let me know what else I can clarify.</p>