drupal’s search abilities are rather limited. Apparently searching in topics works only for main-posts only. I don’t see how this could be enabled for forum replies. Maybe you can spot any plugin which will provide that feature over here: http://drupal.org/project/modules – We’re running Drupal5
About the speed …. it is what it is. IIRC Drupal 7 was about to use Lucene as an optional search engine which would be a major pefromance boost… Let’s keep an eye on that.
Have fun at Balticon!