Processing Load very slow takes more than 30 seconds

Hello, I have been trying to load the content of a single query for days, I have tried both from the server side and directly and both do not work take the same.

I have incorporated the "scroller" plugin but it takes the same, someone can help me, I leave the code here, use php and codeigniter 3.

JS

var table = $('#datatabletransaction').DataTable({
    "deferRender":    true,
    "scrollY":        500,
    "scroller":       true,
    "responsive": true,
    "processing": true,
    "fixedHeader": true,
    "serverSide":true,
        "lengthMenu": [[5, 10, 25, 50, -1], [5, 10, 25, 50, "Todos"]],
        "ajax": {
            "url": mostrarcontenido,
            "type": "POST"
    },        
    language": {
          "url": "http://cdn.datatables.net/plug-ins/1.10.16/i18n/Spanish.json"
    },
    "fixedColumns": true,
    "iDisplayLength": 5,
    "order": [[ 2, "desc" ]]
    });

Controller

public function mostrartablatransaccionestelefonos(){
      $this->load->model('tablas_model');
      $data = array();
      $list = $this->tablas_model->mostrar_registros_ventacompra($estado);
      foreach ($list as $person) {
          $row = array();
          $row[] = $person->num_factura;
          $row[] = $person->num_albaran;
          $row[] = $person->transacion;
          $row[] = $person->imei;
          $row[] = $person->nombre;
          $row[] = $person->fecha_registro;
          $row[] = $person->modelo;
          $row[] = $person->marca;
          $row[] = $person->color;
          $row[] = $person->memoria;
          $row[] = $person->precio_iva;
          $row[] = $person->precio_sin_iva;
    
          $output = array(
                  "data" => $data,
          );
          echo json_encode($output);

   }