Optimisation d'une requete jugée lente

$req_privees = " AND RC.NumNant IN (".$serieNum.")";
      }         
                  
      $req = "SELECT RN.NumResion, RC.DateDeb, NomResant, LibResion, DateResion, ConfResion, FactResion , DenoStruc, Tel1Struc "
            ."FROM reservation RN, reservant RT, structure ST, reservcontenant RC "
            ."WHERE RN.NumResant = RT.NumResant AND ConfResion = ".$type." AND RT.NumStruc = ST.NumStruc AND RC.NumResion = RN.NumResion ".$req_privees;