NowPublic.com uses Sphinx for searching and it is really easy to use Sphinx for Drupal and then it's fast, extremely fast. There is no Sphinx search module because I could not find a module worth of code to release. You just need to set up Sphinx according to its documentation, of course the queries are Drupal specific, you just need to JOIN your tables together USING(vid) and don't forget to start the SELECT with nid:

        sql_query_range = SELECT MIN(nid), MAX(nid) FROM node
        sql_range_step = 1000
        sql_query = \
              SELECT n.nid, n.title, body, changed \
              FROM node n \
              INNER JOIN node_revisions USING(vid) \
              WHERE n.nid BETWEEN $start AND $end AND status = 1

Here is how you add terms: sql_attr_multi = uint tid from query;SELECT nid, tid FROM term_node. Once Sphinx is set up, you need something like the following:

      require_once ('./sphinxapi.php');
      $start_from = $_GET['page'] ? $_GET['page'] : 0;
      $index = ...;
      $page_increment = 10;  // number of nodes shown
      $amount = 10;
      $sphinx = new SphinxClient ();
      $sphinx->SetServer ('localhost', 3312);
      $sphinx->SetWeights (array(100, 1));
      $sphinx->SetMatchMode (SPH_MATCH_ALL);
      $sphinx->SetLimits($start_from * $page_increment, $page_increment);
      $sphinx->SetSortMode(SPH_SORT_TIME_SEGMENTS, 'changed');
      // you can add filters with $sphinx->SetFilter etc
      $result = $sphinx->Query(implode(' ', $keys), $index);
      if (isset($result) && $result !== FALSE && !empty($result['matches'])) {
        $GLOBALS['pager_page_array'][] = $start_from;
        $GLOBALS['pager_total'][] = intval($result['total'] / $page_increment) + 1;
        $GLOBALS['pager_total_items'][] = $result['total'];
        foreach (array_keys($result["matches"]) as $nid) {
          $node = node_load($nid);

While I am unaware of any documentation of the PHP API, the Perl docs work equally well and there is a sample php in the package.