How to speed up performance of taxonomy hierarchical select drupal
Using High-performance JavaScript callback handler will speed up it a bit while i recommend using Simple hierarchical select rather than Hierarchical Select , cause it is much faster in queries (off course less user friendly) .

this is the description High-performance JavaScript callback handler :


  JavaScript callback handler is an interim solution for high-performance server requests including (but not limited to) AHAH, AJAX, JSON, XML, etc.
  This project targets module developers and provides a "bare bone" callback handler which is intended to be addressed by modules wanting to improve response times for specialized tasks.