title: Heroku Rails 4 assets:precompile compiles all (also unchanged) assets
body: <p>On every push to Heroku, all assets are precompiled, so not only assets that have changed. Asset precompilation thus takes more than 250 seconds. I tried all suggestions I found online, but I can't seem to fix it.</p>
  
  <p>I have installed the <code>rails_12factor</code> gem as suggested by Heroku. Static assets are hosted on Cloudfront. We use Bower-Rails for front-end dependencies. Unicorn is the server on Heroku.</p>
  
  <p>These are some settings in the production.rb environment:</p>
  
  <pre><code>config.serve_static_assets = true
  config.action_dispatch.x_sendfile_header = 'X-Accel-Redirect'
  config.assets.compile = true
  config.static_cache_control = 'public, max-age=31536000'
  config.assets.compress = true
  config.assets.digest = true
  </code></pre>
  
  <p>Has anyone experienced this as well? Or can anyone help me?</p>