title: django-import-export export is slow when there is foreign key
body: <p>when there is foreign key in model, exporting is very slow, when I exclude the foreign there is no problem, it starts to download quickly. What might be the issue here? Thanks!</p>
  
  <p>My code is like,</p>
  
  <p>resources.py</p>
  
  <pre><code>class InvoiceResource(resources.ModelResource):
    class Meta:
        model = Invoices
  </code></pre>
  
  <p>views.py</p>
  
  <pre><code>def export_invoice(request):
    person_resource = InvoiceResource()
    dataset = person_resource.export()
    response = HttpResponse(dataset.xlsx, content_type='application/vnd.ms-excel')
    response['Content-Disposition'] = 'attachment; filename="invoices.xlsx"'
    return response
  </code></pre>
  
  <p>models.py</p>
  
  <pre><code>class Invoices(models.Model):
     store_code = models.ForeignKey(MasterData, on_delete=models.CASCADE, db_column='Store Code', blank=True, null=True, verbose_name='Store Code')
     erp_id = models.CharField(db_column='ERP ID', max_length=200, blank=True, null=True, verbose_name='ERP ID')
     store_name = models.CharField(db_column='Store Name', max_length=200, blank=True, null=True, verbose_name='Store Name')
  </code></pre>