title: File reading making PYQT gui unresponsive
body: <p>I have a PyQt Gui application. After user interaction, a QThread is run,  in which three python threads are created for image file reading, cropping, and then writing the result. The python threads communicate using Queue. </p>
  
  <p>With this setup I/O is around the disk's maximum, but it makes the GUI highly unresponsive. If on the other hand I put the  jobs into separate subprocesses, the GUI does not hang, however file reading speed is significantly reduced. </p>
  
  <p>Is there any way I can speed up I/O using subprocesses?</p>