You just close this like that, are you kidding no investigation at all!
I can tell you the problem is django 3 as i test this by only updating it.
On any filter query with relation and django rest framework serializing foreign key this is 10 times slower than under django 2.
A little investigation on your side is requested. Don't put this under the carpet.
Here are my requirements:
asgiref==3.2.10
astroid==2.4.2
cairocffi==1.1.0
CairoSVG==2.4.2
certifi==2020.6.20
cffi==1.14.1
chardet==3.0.4
cssselect2==0.3.0
DateTimeRange==1.0.0
defusedxml==0.6.0
Django==3.1
django-cleanup==5.0.0
django-cors-headers==3.4.0
django-countries==6.1.2
django-crontab==0.7.1
django-extensions==3.0.5
django-filter==2.3.0
django-maintenance-mode==0.14.0
django-model-utils==4.0.0
django-optimistic-lock==1.0.0
django-stubs==1.5.0
django-unused-media==0.2.0
djangorestframework==3.11.1
djangorestframework-stubs==1.2.0
djangorestframework-xml==2.0.0
dnspython==2.0.0
filelock==3.0.12
fpdf==1.7.2
html5lib==1.1
icalendar==4.0.6
idna==2.10
isort==5.3.2
lazy-object-proxy==1.5.1
mbstrdecoder==1.0.0
mccabe==0.6.1
mongoengine==0.20.0
mypy==0.782
mypy-extensions==0.4.3
packaging==20.4
pdfrw==0.4
Pillow==7.2.0
pipdeptree==1.0.0
psycopg2-binary==2.8.5
py3-validate-email==0.2.9
pycparser==2.20
pylint==2.5.3
pymongo==3.11.0
pyparsing==2.4.7
Pyphen==0.9.5
python-dateutil==2.8.1
python-magic==0.4.18
pytz==2020.1
pyuca==1.2
requests==2.24.0
sentry-sdk==0.16.3
six==1.15.0
sqlparse==0.3.1
swapper==1.1.2.post1
tinycss2==1.0.2
toml==0.10.1
tqdm==4.48.2
typed-ast==1.4.1
typepy==1.1.1
typing==3.7.4.3
typing-extensions==3.7.4.2
urllib3==1.25.10
WeasyPrint==51
webencodings==0.5.1
wrapt==1.12.1
Sincerely