Mongoengine is very slow on large documents compared to native pymongo usage

(See also this StackOverflow question)

I have the following mongoengine model:

class MyModel(Document):
    date = DateTimeField(required = True)
    data_dict_1 = DictField(required = False)
    data_dict_2 = DictField(required = True)

In some cases the document in the DB can be very large (around 5-10MB), and the data_dict fields contain complex nested documents (dict of lists of dicts, etc...).

I have encountered two (possibly related) issues:

1. When I run native pymongo find_one() query, it returns within a second. When I run MyModel.objects.first() it takes 5-10 seconds.
2. When I query a single large document from the DB, and then access its field, it takes 10-20 seconds just to do the following:
   m = MyModel.objects.first()
val = m.data_dict_1.get(some_key)

The data in the object does not contain any references to any other objects, so it is not an issue of objects dereferencing.
I suspect it is related to some inefficiency of the internal data representation of mongoengine, which affects the document object construction as well as fields access.

Start by reproducing the comparison between native pymongo find_one() and MyModel.objects.first() using the large MyModel document described here. Measure document construction separately from access to data_dict_1, then compare the timings and identify the internal path responsible; done means the cause is isolated and a measured improvement or actionable limitation is documented.