Latency to be addressed in django for M2M relationship in ModelAdmin dropdown

I have a group of mailboxes which needs to be populated based on customer login and the domains he owns. Customer:User is 1:1 relationship.

Tried:

views.py:

class MailboxAutocomplete(autocomplete.Select2QuerySetView):
    def get_queryset(self):
        if not self.request.user.is_authenticated:
            return Mailbox.objects.none()

        qs = Mailbox.objects.all()
        # Check if the user is in the 'customers' group
        if self.request.user.groups.filter(name='customers').exists():
            print('customer login in mailbox autocomplete view.......')
            # Filter based on the customer's email
            qs = qs.filter(domain__customer__email=self.request.user.email).only('email')

        elif self.request.user.groups.filter(name__in=['resellers']).exists():
            # Filter based on the reseller's email
            qs = qs.filter(domain__customer__reseller__email=self.request.user.email).only('email')

        if self.q:
            # Further filter based on user input (e.g., email matching)
            qs = qs.filter(email__icontains=self.q)
        print(qs.values('email'))
        return qs

in the apps urls.py: path('mailbox-autocomplete/', views.MailboxAutocomplete.as_view(), name='mailbox-autocomplete'), ]

in models.py:

class GroupMailIdsForm(forms.ModelForm):
    class Meta:
        model = GroupMailIds
        fields = "__all__"
    
    mailboxes = forms.ModelMultipleChoiceField(
        queryset=Mailbox.objects.none(),
        widget=autocomplete.ModelSelect2Multiple(url='mailmanager:mailbox-autocomplete')
    )

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        if self.instance.pk:  # Check if the instance is being updated
            if self.request.user.groups.filter(name='customers').exists():
                self.fields['mailboxes'].queryset = Mailbox.objects.filter(domain__customer__email=self.request.user.email)
            elif self.request.user.groups.filter(name='resellers').exists():
                self.fields['mailboxes'].queryset = Mailbox.objects.filter(domain__customer__reseller__email=self.request.user.email)

in admin.py:

class GroupMailIdsAdmin(ImportExportModelAdmin):
    resource_class = GroupMailIdsResource
    ordering = ('address',)
    filter_horizontal = ('mailboxes',)

and in settings.py:

INSTALLED_APPS = [
    'mailmanager.apps.MailmanagerConfig',
    'admin_confirm',
    'dal',
    'dal_select2',
    'django.contrib.admin',
    'jquery',
    ]
with other required django apps.

1. The autocomplete is not working.
2. Django version 4.2 used
3. django-autocomplete-light==3.11.0

IS there something I am missing. I am trying to solve the latency issue for loading about 200k mailboxes in the dropdown. Can I get the precise code to resolve this problem. I need to display the selected mailboxes, and also make selection easy. I am not good with js. Also tried filter_horizontal which is a very good display but causes latency due to the huge number of mailboxes

class GroupMailIdsAdmin(ImportExportModelAdmin):
    resource_class = GroupMailIdsResource
    ordering = ('address',)
    filter_horizontal = ('mailboxes',)
    form = GroupMailIdsForm