[development] The router and links tables split - what caused it?

Yes, I know they split (Drupal 6, was it?). I seem to remember reading somewhere in code, or near code, documentation that this was not simply for clarity or encapsulation, but performance. If I recall the comment alright, does anyone know what that performance issue was? Were people running, say, college sites, or auction sites, and finding they were bloating the links tables? A more general question - what was the scale of the bloating that caused a database to slow down? Answers to these questions may have a direct impact on the development of my module, http://drupal.org/project/taxonomy_treemenu In response to user discussion I'm considering a complete re-write of the links code. Rob