title: Slow loading Django Admin change/add
body: <p>I'm making a classic single view application, mapping multiple
  datasources.</p>
  
  <p>Django-admin is paginated so there's no impact when I view my list, the problem is when I want to change/add it is .</p>
  
  <p>Using the debug-toolbar my queries look fine, I don't think they take a long time.</p>
  
  <p>I tried to use a suggestion here <a href="https://stackoverflow.com/questions/16755312/django-admin-change-form-load-quite-slow">Django admin change form load quite slow</a> and created a form, but this had no impact.</p>
  
  <p>When is use <code>exclude = ['e_vehicle','e_product']</code> it's no surprise that add/change screens load instantly.</p>
  
  <p>Any thoughts please </p>
  
  <p><a href="https://i.stack.imgur.com/J1yvC.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/J1yvC.png" alt="enter image description here"></a></p>
  
  <p><strong>model.py</strong></p>
  
  <pre><code>class Product_Mapping(Trackable):
      product_mapping_id = models.AutoField(primary_key=True)   
      s_product =  models.OneToOneField(sProduct, on_delete=models.CASCADE)
      e_fund_manager = models.ForeignKey(eManager, models.DO_NOTHING, blank=True, null=True)
      e_product = models.ForeignKey(eProduct, models.DO_NOTHING, blank=True, null=True)
      e_vehicle = models.ForeignKey(eVehicle, models.DO_NOTHING, blank=True, null=True)
  </code></pre>
  
  <p>eManager has around 3K</p>
  
  <p>eProduct has around 17K (has fkey to eManager)</p>
  
  <p>eVehicle has around 25K (has fkey to eProduct)</p>
  
  <p><strong>form.py</strong></p>
  
  <pre><code>class MappingProductForm(forms.ModelForm):
      s_product = forms.ChoiceField(required=False,
                              choices=sProduct.objects.values_list('final_publications_product_id', 'product_name'))
      e_fund_manager = forms.ChoiceField(required=False,
                              choices=eManager.objects.values_list('e_fund_manager_id', 'manager_name'))
      e_product = forms.ChoiceField(required=False,
                              choices=eProduct.objects.values_list('e_product_id', 'product_name'))
      e_vehicle = forms.ChoiceField(required=False,
                              choices=eVehicle.objects.values_list('e_vehicle_id', 'formal_vehicle_name'))
  
      class Meta:
          model = Product_Mapping
          fields = '__all__'
  </code></pre>
  
  <p><strong>admin.py</strong></p>
  
  <pre><code>@admin.register(Product_Mapping)
  class ChampProductMappingAdmin(admin.ModelAdmin): 
      form = MappingProductForm
  </code></pre>