Custom Wp_query loop takes very long

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

global $wpdb;
      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.