title: dask computation not executing in parallel
body: <p>I have a directory of json files that I am trying to convert to a dask DataFrame and save it to castra.
  There are 200 files containing O(10**7) json records between them.
  The code is very simple largely following tutorial examples.</p>
  
  <pre><code>import dask.dataframe as dd
  import dask.bag as db
  import json
  txt = db.from_filenames('part-*.json')
  js = txt.map(json.loads)
  df = js.to_dataframe()
  cs=df.to_castra("data.castra")
  </code></pre>
  
  <p>I am running it on a 32 core machine, but the code only utilizes one core at 100%.
  My understanding from the docs is that this code execute in parallel.
  Why is it not?
  Did I misunderstand something?</p>