A developer by the name of Wesley Tanaka worked on a Drupal 6 site that involved millions of content records, presumably imported from another CMS. During the development he discovered a few issues which hampered Drupal's search maintenance speed and wrote up a wonderful article explaining how to boost performance. The suggestions, particularly the first one for boosting reindexing speeds, would not only help sites with massive quantities of data but also sites ran on shared hosts where Drupal can normally be a bit of a resource hog, so I'm hoping they get applied to a future release.