joinAdd question

It´s the folow: In this query, i get 5000 rows from 1o reg, ok. The benchmark is +- 7 seconds; : )

       $db = new DataObject_Frm_usuario();
       $total = $db->count();
       $db->limit(1  ,  5000);
       $cargo = new DataObject_Srh_cargo();
       $db->joinAdd($cargo);
       $lotacao = new DataObject_Srh_lotacao();
       $db->joinAdd($lotacao);

In this query, i get only ONE (1) row from 1o reg ok. The benchmark is +- 7 seconds; : |

       $db = new DataObject_Frm_usuario();
       $total = $db->count();
       $db->limit(5000  ,   1);
       $cargo = new DataObject_Srh_cargo();
       $db->joinAdd($cargo);
       $lotacao = new DataObject_Srh_lotacao();
       $db->joinAdd($lotacao);

And if query start from 15.000 the time is +- 18 seconds.... : (

       $db = new DataObject_Frm_usuario();
       $total = $db->count();
       $db->limit(15000  ,   1);
       $cargo = new DataObject_Srh_cargo();
       $db->joinAdd($cargo);
       $lotacao = new DataObject_Srh_lotacao();
       $db->joinAdd($lotacao);

Why this happens? I don´t use the Pager with joinAdd? Thanks for any help! -- No virus found in this outgoing message. Checked by AVG Anti-Virus. Version: 7.0.289 / Virus Database: 265.4.7 - Release Date: 7/12/2004