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Render-Blocking Script Bug

Spot why this head script blocks rendering and slows the page.

Codehtml
<!DOCTYPE html>
<html>
  <head>
    <title>Product Page</title>
    <!-- 250KB analytics + app bundle -->
    <script src="/js/app.bundle.js"></script>
  </head>
  <body>
    <h1>Welcome</h1>
    <p>Big content here...</p>
  </body>
</html>

What is the performance bug in this HTML?