Custom Wp_query loop takes very long

I found a solution to my question. I used the following code:

global $wpdb;
if($t_query->have_posts()){
   while($t_query->have_posts()){
      $t_query->the_post();
      echo $wpdb->last_query."\r\n";
   }
}

and it showed there was an action somewhere hooked to the_post hook, sending requests to update some meta every time a post was processed.

So, the answer to my question is: I should have used $wpdb->last_query inside the loop to find out what was going on.