Django CRUD operations for multiple records - transaction.atomic vs bulk_create

I have a Django 1.10 model, and a simple model:

I have a simple REST endpoint designed for testing purposes:

@api_view(['POST'])
@transaction.atomic
def r_test(request):
    for record in request.data:
        serializer = TestSerializer(data = record)
        if serializer.is_valid():
           serializer.save()

...that takes 9 seconds (too slow) to execute for one 100 records.

If I rewrite it the following way, it executes instantly.

@api_view(['POST'])
def r_test(request):
    obj_list = []
    for record in request.data:
       obj = Test(field1 = record['field1'])
       obj_list.append(obj)
    Test.objects.bulk_create(obj_list)

What bothers me is that I have read in many sources that wrapping function into a transaction (which I do by adding a decorator @transaction.atomic) would significantly improve insert operations in case of multiple operations. But I can't see this now.

So the question is, does only bulk_create() deliver super-fast speed for inserting big data, or there's something I am doing wrong with transaction.atomic?

Update: Moreover, I have ATOMIC_REQUESTS set to True in my settings. BTW, could it be that something is wrong in the settings? Like, say, Debug = True hinders Django from executing queries in a single transaction ?

Update 2 I have tried both ways with decorator and by wrapping the for loop in with transaction.atomic():. And still I observe instant execution only with bulk_create()

Update 3. My DB is MySQL