title: How can I improve performance of TextIO or AvroIO when reading a very large number of files?
body: <p><code>TextIO.read()</code> and <code>AvroIO.read()</code> (as well as some other Beam IO's) by default don't perform very well in current Apache Beam runners when reading a filepattern that expands into a very large number of files - for example, 1M files.</p>
  
  <p>How can I read such a large number of files efficiently?</p>