While developing a real estate site for one of our clients we tried to import the complete world cities database into the taxonomy (that was the client's demand).
As it happened, Hierarchical Select has failed to digest this big volumes of data and that resulted in the blank white screen popping up.
Drupal developers did confirm the module's inability to work with massive volumes, as a proof link see a comment #1:
https://www.drupal.org/node/544324
In the end of the day, we wrote our own module, that's tuned for processing big amounts of data and is optimized to manage that load.