View Query Alter in Drupal to dramatically increase the query performance. This converts left joins to inner join and disables distinct query.

views_performance_improve.php

<?php                                                                          
/**                                                                             
 * Implements hook_views_query_alter().                                         
 *                                                                              
 * This disables distinct and turns LEFT joins into INNER joins. This increases  
 * performance.                                                       
 */                                                                             
function my_module_views_query_alter(&$view, &$query) {                         
  if ($view->name == "my_view") {                                               
    $query->table_queue['content_type_article']['join']->type = 'INNER';         
    $query->distinct = 0;                                                       
  }                                                                             
}